Golf ball

ABSTRACT

A golf ball  2  includes a spherical core  4  and a cover  6  covering the core  4 . The cover  6  includes an inner cover  8  and an outer cover  10  positioned outside the inner cover  8 . When distances (%) from a central point of the core  4  to nine points and hardnesses at the nine points, which nine points are obtained by dividing a region from the central point to a surface of the core  4  at intervals of 12.5% of a radius of the core  4 , are plotted in a graph, R 2  of a linear approximation curve obtained by a least-square method is 0.95 or greater. A hardness of an innermost layer of the cover  6  is greater than a surface hardness of the core  4 . Preferably, the core is obtained by a rubber composition being crosslinked. The rubber composition includes an acid and/or a salt (d).

This application claims priority on Patent Application No. 2012-126149filed in JAPAN on Jun. 1, 2012 and Patent Application No. 2012-126211filed in JAPAN on Jun. 1, 2012. The entire contents of these JapanesePatent Applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to golf balls. Specifically, the presentinvention relates to golf balls that include a solid core and a coverincluding two or more layers.

2. Description of the Related Art

Golf players' foremost requirement for golf balls is flight performance.In particular, golf players place importance on flight performance uponshots with a driver and a long iron. Flight performance correlates withthe resilience performance of a golf ball. When a golf ball havingexcellent resilience performance is hit, the golf ball flies at a highspeed, thereby achieving a large flight distance. Golf balls thatinclude a core having excellent resilience performance are disclosed inJP61-37178, JP2008-212681 (US2008/0214324), JP2008-523952(US2006/0135287 and US2007/0173607), and JP2009-119256 (US2009/0124757).

The core disclosed in JP61-37178 is obtained from a rubber compositionthat includes a co-crosslinking agent and a crosslinking activator. Thispublication discloses palmitic acid, stearic acid, and myristic acid asthe crosslinking activator.

The core disclosed in JP2008-212681 is obtained from a rubbercomposition that includes an organic peroxide, a metal salt of anα,β-unsaturated carboxylic acid, and a copper salt of a fatty acid.

The core disclosed in JP2008-523952 is obtained from a rubbercomposition that includes a metal salt of an unsaturated monocarboxylicacid, a free radical initiator, and a non-conjugated diene monomer.

The core disclosed in JP2009-119256 is obtained from a rubbercomposition that includes a polybutadiene whose vinyl content is equalto or less than 2%, whose cis 1,4-bond content is equal to or greaterthan 80%, and which has an active end modified with an alkoxysilanecompound.

An appropriate trajectory height is required in order to achieve a largeflight distance. A trajectory height depends on a spin rate and a launchangle. With a golf ball that achieves a high trajectory by a high spinrate, a flight distance is insufficient. With a golf ball that achievesa high trajectory by a high launch angle, a large flight distance isobtained. Use of an outer-hard/inner-soft structure in a golf ball canachieve a low spin rate and a high launch angle. Modifications regardinga hardness distribution of a core are disclosed in J26-154357 (U.S. Pat.No. 5,403,010), JP2008-194471 (U.S. Pat. No. 7,344,455, US2008/0194358,US2008/0194359, and US2008/0214325), and JP2008-194473 (US2008/0194357and US2008/0312008). In the core disclosed in JP6-154357, a JIS-Chardness H1 at the central point of the core is 58 to 73, a JIS-Chardness H2 in a region that extends over a distance range from equal toor greater than 5 mm to equal to or less than 10 mm from the centralpoint is equal to or greater than 65 but equal to or less than 75, aJIS-C hardness H3 at a point located at a distance of 15 mm from thecentral point is equal to or greater than 74 but equal to or less than82, and a JIS-C hardness H4 at the surface of the core is equal to orgreater than 76 but equal to or less than 84. The hardness H2 is greaterthan the hardness H1, the hardness H3 is greater than the hardness H2,and the hardness H4 is equal to or greater than the hardness H3.

In the core disclosed in JP2008-194471, a Shore D hardness at thecentral point of the core is equal to or greater than 30 but equal to orless than 48, a Shore D hardness at a point located at a distance of 4mm from the central point is equal to or greater than 34 but equal to orless than 52, a Shore D hardness at a point located at a distance of 8mm from the central point is equal to or greater than 40 but equal to orless than 58, a Shore D hardness at a point located at a distance of 12mm from the central point is equal to or greater than 43 but equal to orless than 61, a Shore D hardness in a region that extends over adistance range from equal to or greater than 2 mm to equal to or lessthan 3 mm from the surface of the core is equal to or greater than 36but equal to or less than 54, and a Shore D hardness at the surface isequal to or greater than 41 but equal to or less than 59.

In the core disclosed in JP2008-194473, a Shore D hardness at thecentral point of the core is equal to or greater than 25 but equal to orless than 45, a Shore D hardness in a region that extends over adistance range from equal to or greater than 5 mm to equal to or lessthan 10 mm from the central point is equal to or greater than 39 butequal to or less than 58, a Shore D hardness at a point located at adistance of 15 mm from the central point is equal to or greater than 36but equal to or less than 55, and a Shore D hardness at the surface ofthe core is equal to or greater than 55 but equal to or less than 75.

JP2010-253268 (US2010/0273575) discloses a golf ball that includes acore, an envelope layer, a mid layer, and a cover. In the core, thehardness gradually increases from the central point of the core to thesurface of the core. The difference between a JIS-C hardness at thesurface and a JIS-C hardness at the central point is equal to or greaterthan 15. The hardness of the cover is greater than the hardness of themid layer, and the hardness of the mid layer is greater than thehardness of the envelope layer.

For a tee shot on a par-three hole and a second shot on a par-four hole,a middle iron is frequently used. Golf players also desire a largeflight distance upon a shot with a middle iron. An object of the presentinvention is to provide a golf ball that exerts excellent flightperformance upon a shot with a middle iron.

Golf players' requirements for flight distance have been escalated morethan ever. Golf players further desire golf balls having excellent feelat impact. Golf balls that satisfy in terms of flight distance tend tohave inferior feel at impact.

Another object of the present invention is to provide a golf ball thatachieves a large flight distance and excellent feel at impact upon ashot with a driver.

SUMMARY OF THE INVENTION

A golf ball according to the present invention includes a spherical coreand a cover covering the core and including two or more layers. Whendistances (%) from a central point of the core to nine points and JIS-Chardnesses at the nine points, which nine points are obtained bydividing a region from the central point of the core to a surface of thecore at intervals of 12.5% of a radius of the core, are plotted in agraph, R² of a linear approximation curve obtained by a least-squaremethod is equal to or greater than 0.95. A JIS-C hardness Hi of aninnermost layer of the cover is greater than a JIS-C hardness Hs at thesurface of the core. In the golf ball according to the presentinvention, a hardness distribution is appropriate. In the golf ball, theenergy loss is low when the golf ball is hit with a middle iron. Whenthe golf ball is hit with a middle iron, the spin rate is low. The lowspin rate achieves a large flight distance.

According to another aspect, a golf ball according to the presentinvention includes a spherical core and a cover covering the core andincluding two or more layers. When distances (%) from a central point ofthe core to nine points and JIS-C hardnesses at the nine points, whichnine points are obtained by dividing a region from the central point ofthe core to a surface of the core at intervals of 12.5% of a radius ofthe core, are plotted in a graph, R² of a linear approximation curveobtained by a least-square method is equal to or greater than 0.95. AJIS-C hardness Hi of an innermost layer of the cover is equal to or lessthan a JIS-C hardness Hs at the surface of the core. In the golf ballaccording to the present invention, a hardness distribution isappropriate. In the golf ball, the energy loss is low when the golf ballis hit with a driver. When the golf ball is hit with a driver, the spinrate is low. The low spin rate achieves a large flight distance. In thegolf ball, the innermost layer achieves soft feel at impact. The golfball has both excellent flight performance and excellent feel at impact.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially cutaway cross-sectional view of a golf ballaccording to a first embodiment of the present invention;

FIG. 2 is a line graph showing a hardness distribution of a core of thegolf ball in FIG. 1;

FIG. 3 is a partially cutaway cross-sectional view of a golf ballaccording to a second embodiment of the present invention; and

FIG. 4 is a line graph showing a hardness distribution of a core of thegolf ball in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following will describe in detail the present invention, based onpreferred embodiments with reference to the accompanying drawings.

First Embodiment

A golf ball 2 shown in FIG. 1 includes a spherical core 4 and a cover 6covering the core 4. The cover 6 includes an inner cover 8 and an outercover 10 positioned outside the inner cover 8. The inner cover 8 is aninnermost layer of the cover 6. The outer cover 10 is an outermost layerof the cover 6. The cover 6 may include another one or more layersbetween the inner cover 8 and the outer cover 10. On the surface of theouter cover 10, a large number of dimples 12 are formed. Of the surfaceof the golf ball 2, a part other than the dimples 12 is a land 14. Thegolf ball 2 includes a paint layer and a mark layer on the external sideof the outer cover 10, but these layers are not shown in the drawing.

The golf ball 2 has a diameter of 40 mm or greater but 45 mm or less.From the standpoint of conformity to the rules established by the UnitedStates Golf Association (USGA), the diameter is preferably equal to orgreater than 42.67 mm. In light of suppression of air resistance, thediameter is preferably equal to or less than 44 mm and more preferablyequal to or less than 42.80 mm. The golf ball 2 has a weight of 40 g orgreater but 50 g or less. In light of attainment of great inertia, theweight is preferably equal to or greater than 44 g and more preferablyequal to or greater than 45.00 g. From the standpoint of conformity tothe rules established by the USGA, the weight is preferably equal to orless than 45.93 g.

FIG. 2 is a line graph showing a hardness distribution of the core 4 ofthe golf ball 2 in FIG. 1. The horizontal axis of the graph indicatesthe ratio (%) of a distance from the central point of the core 4 to theradius of the core 4. The vertical axis of the graph indicates a JIS-Chardness. Nine measuring points obtained by dividing a region from thecentral point of the core 4 to the surface of the core 4 at intervals of12.5% of the radius of the core 4 are plotted in the graph. The ratio ofthe distance from the central point of the core 4 to each of thesemeasuring points to the radius of the core 4 is as follows.

First point: 0.0% (central point)

Second point: 12.5%

Third point: 25.0%

Fourth point: 37.5%

Fifth point: 50.0%

Sixth point: 62.5%

Seventh point: 75.0%

Eighth point: 87.5%

Ninth point: 100.0% (surface)

Hardnesses at the first to eighth points are measured by pressing aJIS-C type hardness scale against a cut plane of the core 4 that hasbeen cut into two halves. A hardness Hs at the ninth point is measuredby pressing the JIS-C type hardness scale against the surface of thespherical core 4. For the measurement, an automated rubber hardnessmeasurement machine (trade name “P1”, manufactured by Kobunshi KeikiCo., Ltd.), to which this hardness scale is mounted, is used. In thepresent invention, a JIS-C hardness at a measuring point whose distancefrom the central point of the core 4 is x (%) is represented by H(x).The hardness at the central point of the core 4 is represented by H(0).

FIG. 2 also shows a linear approximation curve obtained by aleast-square method on the basis of the distances and the hardnesses ofthe nine measuring points. As is clear from FIG. 2, the broken line doesnot greatly deviate from the linear approximation curve. In other words,the broken line has a shape close to the linear approximation curve. Inthe core 4, the hardness linearly increases from its central pointtoward its surface. When the core 4 is hit with a middle iron, theenergy loss is low. The core 4 has excellent resilience performance.When the golf ball 2 is hit with a middle iron, the flight distance islarge.

In the core 4, R² of the linear approximation curve obtained by theleast-square method is equal to or greater than 0.95. R² is an indexindicating the linearity of the broken line. For the core 4 for which R²is equal to or greater than 0.95, the shape of the broken line of thehardness distribution is close to a straight line. The core 4 for whichR² is equal to or greater than 0.95 has excellent resilienceperformance. R² is more preferably equal to or greater than 0.96 andparticularly preferably equal to or greater than 0.97. R² is calculatedby squaring a correlation coefficient R. The correlation coefficient Ris calculated by dividing the covariance of the distance (%) from thecentral point and the hardness (JIS-C) by the standard deviation of thedistance (%) from the central point and the standard deviation of thehardness (JIS-C).

The core 4 is obtained by crosslinking a rubber composition. The rubbercomposition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) an acid and/or a salt.

During heating and forming of the core 4, the base rubber (a) iscrosslinked by the co-crosslinking agent (b). The heat of thecrosslinking reaction remains near the central point of the core 4.Thus, during heating and forming of the core 4, the temperature at thecentral portion is high. The temperature gradually decreases from thecentral point toward the surface. It is inferred that in the rubbercomposition, the acid reacts with a metal salt of the co-crosslinkingagent (b) to bond to cation. It is inferred that in the rubbercomposition, the salt reacts with the metal salt of the co-crosslinkingagent (b) to exchange cation. By the bonding and the exchange, metallicbonding is broken. This breaking is likely to occur in the centralportion of the core 4 where the temperature is high, and is unlikely tooccur near the surface of the core 4. As a result, the crosslinkingdensity of the core 4 increases from its central point toward itssurface. In the core 4, an outer-hard/inner-soft structure can beachieved. Furthermore, when the rubber composition includes an organicsulfur compound (e) together with the acid and/or the salt (d), thegradient of the hardness distribution can be controlled, and the degreeof the outer-hard/inner-soft structure of the core 4 can be increased.When the golf ball 2 that includes the core 4 is hit with a middle iron,the spin rate is low. In the golf ball 2, excellent flight performanceis achieved upon a shot with a middle iron.

Examples of the base rubber (a) of the core 4 include polybutadienes,polyisoprenes, styrene-butadiene copolymers, ethylene-propylene-dienecopolymers, and natural rubbers. In light of resilience performance,polybutadienes are preferred. When a polybutadiene and another rubberare used in combination, it is preferred that the polybutadiene isincluded as a principal component. Specifically, the proportion of thepolybutadiene to the entire base rubber is preferably equal to orgreater than 50% by weight and more preferably equal to or greater than80% by weight. The proportion of cis-1,4 bonds in the polybutadiene ispreferably equal to or greater than 40% by weight and more preferablyequal to or greater than 80% by weight.

A polybutadiene in which the proportion of 1,2-vinyl bonds is equal toor less than 2.0% by weight is preferred. The polybutadiene cancontribute to the resilience performance of the core 4. In this respect,the proportion of 1,2-vinyl bonds is preferably equal to or less than1.7% by weight and particularly preferably equal to or less than 1.5% byweight.

From the standpoint that a polybutadiene having a low proportion of1,2-vinyl bonds and excellent polymerization activity is obtained, arare-earth-element-containing catalyst is preferably used for synthesisof a polybutadiene. In particular, a polybutadiene synthesized with acatalyst containing neodymium, which is a lanthanum-series rare earthelement compound, is preferred.

The polybutadiene has a Mooney viscosity (ML₁₊₄(100° C.)) of preferably30 or greater, more preferably 32 or greater, and particularlypreferably 35 or greater. The Mooney viscosity (ML₁₊₄(100° C.)) ispreferably equal to or less than 140, more preferably equal to or lessthan 120, even more preferably equal to or less than 100, andparticularly preferably equal to or less than 80. The Mooney viscosity(ML₁₊₄ (100° C.)) is measured according to the standards of “JIS K6300”.The measurement conditions are as follows.

Rotor: L rotor

Preheating time: 1 minute

Rotating time of rotor: 4 minutes

Temperature: 100° C.

In light of workability, the polybutadiene has a molecular weightdistribution (Mw/Mn) of preferably 2.0 or greater, more preferably 2.2or greater, even more preferably 2.4 or greater, and particularlypreferably 2.6 or greater. In light of resilience performance, themolecular weight distribution (Mw/Mn) is preferably equal to or lessthan 6.0, more preferably equal to or less than 5.0, even morepreferably equal to or less than 4.0, and particularly preferably equalto or less than 3.4. The molecular weight distribution (Mw/Mn) iscalculated by dividing the weight average molecular weight Mw by thenumber average molecular weight Mn.

The molecular weight distribution is measured by gel permeationchromatography(“HLC-8120GPC” manufactured by Tosoh Corporation). Themeasurement conditions are as follows.

Detector: differential refractometer

Column: GMHHXL (manufactured by Tosoh Corporation)

Column temperature: 40° C.

Mobile phase: tetrahydrofuran

The molecular weight distribution is calculated as a value obtained byconversion using polystyrene standard.

The co-crosslinking agent (b) is:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.

The rubber composition may include only the α,β-unsaturated carboxylicacid (b1) or only the metal salt (b2) of the α,β-unsaturated carboxylicacid as the co-crosslinking agent (b). The rubber composition mayinclude both the α,β-unsaturated carboxylic acid (b1) and the metal salt(b2) of the α,β-unsaturated carboxylic acid as the co-crosslinking agent(b).

The metal salt (b2) of the α,β-unsaturated carboxylic acidgraft-polymerizes with the molecular chain of the base rubber, therebycrosslinking the rubber molecules. When the rubber composition includesthe α,β-unsaturated carboxylic acid (b1), the rubber compositionpreferably further includes a metal compound (f). The metal compound (f)reacts with the α,β-unsaturated carboxylic acid (b1) in the rubbercomposition. A salt obtained by this reaction graft-polymerizes with themolecular chain of the base rubber.

Examples of the metal compound (f) include metal hydroxides such asmagnesium hydroxide, zinc hydroxide, calcium hydroxide, sodiumhydroxide, lithium hydroxide, potassium hydroxide, and copper hydroxide;metal oxides such as magnesium oxide, calcium oxide, zinc oxide, andcopper oxide; and metal carbonates such as magnesium carbonate, zinccarbonate, calcium carbonate, sodium carbonate, lithium carbonate, andpotassium carbonate. A compound that includes a bivalent metal ispreferred. The compound that includes the bivalent metal reacts with theco-crosslinking agent (b) to form metal crosslinks. The metal compound(f) is particularly preferably a zinc compound. Two or more metalcompounds may be used in combination.

Examples of the α,β-unsaturated carboxylic acids include acrylic acid,methacrylic acid, fumaric acid, maleic acid, and crotonic acid. Examplesof the metal component in the metal salt (b2) of the α,β-unsaturatedcarboxylic acid include sodium ion, potassium ion, lithium ion,magnesium ion, calcium ion, zinc ion, barium ion, cadmium ion, aluminumion, tin ion, and zirconium ion. The metal salt (b2) of theα,β-unsaturated carboxylic acid may include two or more types of ions.From the standpoint that metal crosslinks are likely to occur betweenthe rubber molecules, bivalent metal ions such as magnesium ion, calciumion, zinc ion, barium ion, and cadmium ion are preferred. The metal salt(b2) of the α,β-unsaturated carboxylic acid is particularly preferablyzinc acrylate.

In light of resilience performance of the golf ball 2, the amount of theco-crosslinking agent (b) is preferably equal to or greater than 15parts by weight and particularly preferably equal to or greater than 20parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact, the amount is preferably equal to or less than 50 partsby weight, more preferably equal to or less than 45 parts by weight, andparticularly preferably equal to or less than 40 parts by weight, per100 parts by weight of the base rubber.

The crosslinking initiator (c) is preferably an organic peroxide. Theorganic peroxide contributes to the resilience performance of the golfball 2. Examples of preferable organic peroxides include dicumylperoxide,

1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Inlight of versatility, dicumyl peroxide is preferred.

In light of resilience performance of the golf ball 2, the amount of thecrosslinking initiator (c) is preferably equal to or greater than 0.2parts by weight and particularly preferably equal to or greater than 0.5parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact and durability of the golf ball 2, the amount ispreferably equal to or less than 5.0 parts by weight and particularlypreferably equal to or less than 2.5 parts by weight, per 100 parts byweight of the base rubber.

In the present invention, the co-crosslinking agent (b) is not includedin the concept of the acid and/or the salt (d). It is inferred that asdescribed above, the acid and/or the salt (d) breaks the metalcrosslinks by the co-crosslinking agent (b) in the central portion ofthe core 4 during heating and forming of the core 4. Examples of theacid and/or the salt (d) include oxo acids, such as carboxylic acids,sulfonic acids, and phosphoric acid, and salts thereof; and hydroacids,such as hydrochloric acid and hydrofluoric acid, and salts thereof. Oxoacids and salts thereof are preferred. A carboxylic acid and/or a saltthereof (d1) is more preferred. Carboxylates are particularly preferred.

The carboxylic acid component of the carboxylic acid and/or the saltthereof (d1) has a carboxyl group. The carbon number of the carboxylicacid component of the carboxylic acid and/or the salt thereof (d1) ispreferably equal to or greater than 1 but equal to or less than 30, morepreferably equal to or greater than 3 but equal to or less than 30, andeven more preferably equal to or greater than 5 but equal to or lessthan 28. Examples of the carboxylic acid include aliphatic carboxylicacids (fatty acids) and aromatic carboxylic acids. Fatty acids and saltsthereof are preferred.

The rubber composition may include a saturated fatty acid or a saltthereof, or may include an unsaturated fatty acid or a salt thereof. Thesaturated fatty acid and the salt thereof are preferred.

Examples of fatty acids include butyric acid (C4), valeric acid (C5),caproic acid (C6), enanthic acid (C7), caprylic acid (octanoic acid)(C8), pelargonic acid (C9), capric acid (C10), lauric acid (C12),myristic acid (C14), myristoleic acid (C14), pentadecylic acid (C15),palmitic acid (C16), palmitoleic acid (C16), margaric acid (C17),stearic acid (C18), elaidic acid (C18), vaccenic acid (C18), oleic acid(C18), linolic acid (C18), linolenic acid (C18), 12-hydroxystearic acid(C18), arachidic acid (C20), gadoleic acid (C20), arachidonic acid(C20), eicosenoic acid (C20), behenic acid (C22), erucic acid (C22),lignoceric acid (C24), nervonic acid (C24), cerotic acid (C26), montanicacid (C28), and melissic acid (C30). Two or more fatty acid salts may beused in combination. Octanoic acid, lauric acid, myristic acid, palmiticacid, stearic acid, oleic acid, and behenic acid are preferred.

An aromatic carboxylic acid has an aromatic ring and a carboxyl group.Examples of aromatic carboxylic acids include benzoic acid, phthalicacid, isophthalic acid, terephthalic acid, hemimellitic acid(benzene-1,2,3-tricarboxylic acid), trimellitic acid(benzene-1,2,4-tricarboxylic acid), trimeric acid(benzene-1,3,5-tricarboxylic acid), mellophanic acid(benzene-1,2,3,4-tetracarboxylic acid), prehnitic acid(benzene-1,2,3,5-tetracarboxylic acid), pyromellitic acid(benzene-1,2,4,5-tetracarboxylicacid), mellitic acid(benzenehexacarboxylic acid), diphenic acid (biphenyl-2,2′-dicarboxylic acid),toluic acid (methylbenzoic acid), xylic acid, prehnitylic acid(2,3,4-trimethylbenzoic acid), γ-isodurylic acid (2,3,5-trimethylbenzoicacid), durylic acid (2,4,5-trimethylbenzoic acid), β-isodurylic acid(2,4,6-trimethylbenzoic acid), α-isodurylic acid (3,4,5-trimethylbenzoicacid), cuminic acid (4-isopropylbenzoic acid), uvitic acid(5-methylisophthalic acid), α-toluic acid (phenylacetic acid),hydratropic acid (2-phenylpropanoic acid), and hydrocinnamic acid(3-phenylpropanoic acid).

The rubber composition may include a salt of an aromatic carboxylic acidsubstituted with a hydroxyl group, an alkoxy group, or an oxo group.Examples of this carboxylic acid can include salicylic acid(2-hydroxybenzoic acid), anisic acid (methoxybenzoic acid), cresotinicacid (hydroxy(methyl) benzoic acid), o-homosalicylic acid(2-hydroxy-3-methylbenzoic acid), m-homosalicylic acid(2-hydroxy-4-methylbenzoic acid), p-homosalicylic acid(2-hydroxy-5-methylbenzoic acid), o-pyrocatechuic acid(2,3-dihydroxybenzoic acid), β-resorcylic acid (2,4-dihydroxybenzoicacid), γ-resorcylic acid (2,6-dihydroxybenzoic acid), protocatechuicacid (3,4-dihydroxybenzoic acid), α-resorcylic acid(3,5-dihydroxybenzoic acid), vanillic acid (4-hydroxy-3-methoxybenzoicacid), isovanillic acid (3-hydroxy-4-methoxybenzoic acid), veratric acid(3,4-dimethoxybenzoic acid), o-veratric acid (2,3-dimethoxybenzoicacid), orsellinic acid (2,4-dihydroxy-6-methylbenzoic acid), m-hemipinicacid (4,5-dimethoxyphthalic acid), gallic acid (3,4,5-trihydroxybenzoicacid), syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid), asaronicacid (2,4,5-trimethoxybenzoic acid), mandelic acid(hydroxy(phenyl)acetic acid), vanillylmandelic acid(hydroxy(4-hydroxy-3-methoxyphenyl)acetic acid), homoanisic acid((4-methoxyphenyl)acetic acid), homogentisic acid((2,5-dihydroxyphenyl)acetic acid), homoprotocatechuic acid((3,4-dihydroxyphenyl)acetic acid), homovanillic acid((4-hydroxy-3-methoxyphenyl)acetic acid), homoisovanillic acid((3-hydroxy-4-methoxyphenyl)acetic acid), homoveratric acid((3,4-dimethoxyphenyl)acetic acid), o-homoveratric acid((2,3-dimethoxyphenyl)acetic acid), homophthalic acid(2-(carboxymethyl)benzoic acid), homoisophthalic acid(3-(carboxymethyl)benzoic acid), homoterephthalic acid(4-(carboxymethyl)benzoic acid), phthalonic acid(2-(carboxycarbonyl)benzoic acid), isophthalonic acid(3-(carboxycarbonyl)benzoic acid), terephthalonic acid(4-(carboxycarbonyl)benzoic acid), benzilic acid (hydroxydiphenylaceticacid), atrolactic acid (2-hydroxy-2-phenylpropanoic acid), tropic acid(3-hydroxy-2-phenylpropanoic acid), melilotic acid(3-(2-hydroxyphenyl)propanoic acid), phloretic acid(3-(4-hydroxyphenyl)propanoic acid), hydrocaffeic acid(3-(3,4-dihydroxyphenyl)propanoic acid), hydroferulic acid(3-(4-hydroxy-3-methoxyphenyl)propanoic acid), hydroisoferulic acid(3-(3-hydroxy-4-methoxyphenyl)propanoic acid), p-coumaric acid(3-(4-hydroxyphenyl)acrylic acid), umbellic acid(3-(2,4-dihydroxyphenyl)acrylic acid), caffeic acid(3-(3,4-dihydroxyphenyl)acrylic acid), ferulic acid(3-(4-hydroxy-3-methoxyphenyl)acrylic acid), isoferulic acid(3-(3-hydroxy-4-methoxyphenyl)acrylic acid), and sinapic acid(3-(4-hydroxy-3,5-dimethoxyphenyl)acrylic acid).

The cationic component of the carboxylate is a metal ion or an organiccation. Examples of the metal ion include sodium ion, potassium ion,lithium ion, silver ion, magnesium ion, calcium ion, zinc ion, bariumion, cadmium ion, copper ion, cobalt ion, nickel ion, manganese ion,aluminum ion, iron ion, tin ion, zirconium ion, and titanium ion. Two ormore types of ions may be used in combination.

The organic cation is a cation having a carbon chain. Examples of theorganic cation include organic ammonium ions. Examples of organicammonium ions include primary ammonium ions such as stearylammonium ion,hexylammonium ion, octylammonium ion, and 2-ethylhexylammonium ion;secondary ammonium ions such as dodecyl (lauryl)ammoniumion, andoctadecyl (stearyl)ammonium ion; tertiary ammonium ions such astrioctylammonium ion; and quaternary ammonium ions such asdioctyldimethylammonium ion, and distearyldimethylammonium ion. Two ormore types of organic cations may be used in combination.

Examples of preferable carboxylates include a potassium salt, amagnesium salt, an aluminum salt, a zinc salt, an iron salt, a coppersalt, a nickel salt, or a cobalt salt of octanoic acid, lauric acid,myristic acid, palmitic acid, stearic acid, oleic acid, or behenic acid.Zinc salts of carboxylic acids are particularly preferred. Specificexamples of preferable carboxylates include zinc octoate, zinc laurate,zinc myristate, and zinc stearate.

In light of linearity of the hardness distribution of the core 4, theamount of the acid and/or the salt (d) is preferably equal to or greaterthan 0.5 parts by weight, more preferably equal to or greater than 1.0parts by weight, even more preferably equal to or greater than 1.5 partsby weight, and particularly preferably equal to or greater than 2.0parts by weight, per 100 parts by weight of the base rubber. In light ofresilience performance, the amount is preferably equal to or less than40 parts by weight, more preferably less than 40 parts by weight, evenmore preferably equal to or less than 30 parts by weight, andparticularly preferably equal to or less than 20 parts by weight, per100 parts by weight of the base rubber.

The weight ratio of the co-crosslinking agent (b) and the acid and/orthe salt (d) in the rubber composition is preferably equal to or greaterthan 3/7 but equal to or less than 9/1, and is particularly preferablyequal to or greater than 4/6 but equal to or less than 8/2. From therubber composition in which this weight ratio is within the above range,the core 4 whose hardness linearly increases from its central pointtoward its surface can be obtained.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zincacrylate whose surface is coated with stearic acid or zinc stearate forthe purpose of improving dispersibility to rubber is present. In thepresent invention, when the rubber composition includes this zincacrylate, this coating material is not included in the concept of theacid and/or the salt (d).

The rubber composition preferably further includes an organic sulfurcompound (e). The organic sulfur compound (e) can contribute to controlof: the linearity of the hardness distribution of the core 4; and thedegree of the outer-hard/inner-soft structure. An example of the organicsulfur compound (e) is an organic compound having a thiol group or apolysulfide linkage having 2 to 4 sulfur atoms. A metal salt of thisorganic compound is also included in the organic sulfur compound (e).Examples of the organic sulfur compound (e) include aliphatic compoundssuch as aliphatic thiols, aliphatic thiocarboxylic acids, aliphaticdithiocarboxylic acids, and aliphatic polysulfides; heterocycliccompounds; alicyclic compounds such as alicyclic thiols, alicyclicthiocarboxylic acids, alicyclic dithiocarboxylic acids, and alicyclicpolysulfides; and aromatic compounds. Specific examples of the organicsulfur compound (e) include thiophenols, thionaphthols, polysulfides,thiocarboxylic acids, dithiocarboxylic acids, sulfenamides, thiurams,dithiocarbamates, and thiazoles. Preferable organic sulfur compounds (e)are thiophenols, polysulfides having 2 to 4 sulfur atoms, thionaphthols,thiurams, and metal salts thereof.

Specific examples of the organic sulfur compound (e) are represented bythe following chemical formulas (1) to (4).

In the chemical formula (1), R1 to R5 each represent H or a substituent.

In the chemical formula (2), R1 to R10 each represent H or asubstituent.

In the chemical formula (3), R1 to R5 each represent H or a substituent,and M1 represents a monovalent metal atom.

In the chemical formula (4), R1 to R10 each represent H or asubstituent, and M2 represents a bivalent metal atom.

In the formulas (1) to (4), each substituent is at least one groupselected from the group consisting of a halogen group (F, Cl, Br, I), analkyl group, a carboxyl group (—COOH), an ester (—COOR) of a carboxylgroup, a formyl group (—CHO), an acyl group (—COR), a carbonyl halidegroup (—COX), a sulfo group (—SO₃H), an ester(—SO₃R) of a sulfo group, asulfonyl halide group (—SO₂X), a sulfino group (—SO₂H), an alkylsulfinylgroup (—SOR), a carbamoyl group (—CONH₂), an alkyl halide group, a cyanogroup (—CN), and an alkoxy group (—OR).

Examples of the organic sulfur compound represented by the chemicalformula (1) include thiophenol; thiophenols substituted with halogengroups, such as 4-fluorothiophenol, 2,5-difluorothiophenol,2,4,5-trifluorothiophenol, 2,4,5,6-tetrafluorothiophenol,pentafluorothiophenol, 2-chlorothiophenol, 4-chlorothiophenol,2,4-dichlorothiophenol, 2,5-dichlorothiophenol,2,4,5-trichlorothiophenol, 2,4,5,6-tetrachlorothiophenol,pentachlorothiophenol, 4-bromothiophenol, 2,5-dibromothiophenol,2,4,5-tribromothiophenol, 2,4,5,6-tetrabromothiophenol,pentabromothiophenol, 4-iodothiophenol, 2,5-diiodothiophenol,2,4,5-triiodothiophenol, 2,4,5,6-tetraiodothiophenol, andpentaiodothiophenol; thiophenols substituted with alkyl groups, such as4-methylthiophenol, 2,4,5-trimethylthiophenol, pentamethylthiophenol,4-t-butylthiophenol, 2,4,5-tri-t-butylthiophenol, andpenta-t-butylthiophenol; thiophenols substituted with carboxyl groups,such as 4-carboxythiophenol, 2,4,6-tricarboxythiophenol, andpentacarboxythiophenol; thiophenols substituted with alkoxycarbonylgroups, such as 4-methoxycarbonylthiophenol,2,4,6-trimethoxycarbonylthiophenol, and pentamethoxycarbonylthiophenol;thiophenols substituted with formyl groups, such as 4-formylthiophenol,2,4,6-triformylthiophenol, and pentaformylthiophenol; thiophenolssubstituted with acyl groups, such as 4-acetylthiophenol,2,4,6-triacetylthiophenol, and pentaacetylthiophenol; thiophenolssubstituted with carbonyl halide groups, such as4-chlorocarbonylthiophenol, 2,4,6-tri(chlorocarbonyl)thiophenol, andpenta(chlorocarbonyl)thiophenol; thiophenols substituted with sulfogroups, such as 4-sulfothiophenol, 2,4,6-trisulfothiophenol, andpentasulfothiophenol; thiophenols substituted with alkoxysulfonylgroups, such as 4-methoxysulfonylthiophenol,2,4,6-trimethoxysulfonylthiophenol, and pentamethoxysulfonylthiophenol;thiophenols substituted with sulfonyl halide groups, such as4-chlorosulfonylthiophenol, 2,4,6-tri(chlorosulfonyl)thiophenol, andpenta(chlorosulfonyl)thiophenol; thiophenols substituted with sulfinogroups, such as 4-sulfinothiophenol, 2,4,6-trisulfinothiophenol, andpentasulfinothiophenol; thiophenols substituted with alkylsulfinylgroups, such as 4-methylsulfinylthiophenol,2,4,6-tri(methylsulfinyl)thiophenol, andpenta(methylsulfinyl)thiophenol; thiophenols substituted with carbamoylgroups, such as 4-carbamoylthiophenol, 2,4,6-tricarbamoylthiophenol, andpentacarbamoylthiophenol; thiophenols substituted with alkyl halidegroups, such as 4-trichloromethylthiophenol,2,4,6-tri(trichloromethyl)thiophenol, andpenta(trichloromethyl)thiophenol; thiophenols substituted with cyanogroups, such as 4-cyanothiophenol, 2,4,6-tricyanothiophenol, andpentacyanothiophenol; and thiophenols substituted with alkoxy groups,such as 4-methoxythiophenol, 2,4,6-trimethoxythiophenol, andpentamethoxythiophenol. Each of these thiophenols is substituted withone type of substituent.

Another example of the organic sulfur compound represented by thechemical formula (1) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol,4-chloro-2-aminothiophenol, 4-chloro-2-hydroxythiophenol,4-chloro-2-phenylthiothiophenol, 4-methyl-2-nitrothiophenol,4-methyl-2-aminothiophenol, 4-methyl-2-hydroxythiophenol,4-methyl-2-phenylthiothiophenol, 4-carboxy-2-nitrothiophenol,4-carboxy-2-aminothiophenol, 4-carboxy-2-hydroxythiophenol,4-carboxy-2-phenylthiothiophenol, 4-methoxycarbonyl-2-nitrothiophenol,4-methoxycarbonyl-2-aminothiophenol,4-methoxycarbonyl-2-hydroxythiophenol,4-methoxycarbonyl-2-phenylthiothiophenol, 4-formyl-2-nitrothiophenol,4-formyl-2-aminothiophenol, 4-formyl-2-hydroxythiophenol,4-formyl-2-phenylthiothiophenol, 4-acetyl-2-nitrothiophenol,4-acetyl-2-aminothiophenol, 4-acetyl-2-hydroxythiophenol,4-acetyl-2-phenylthiothiophenol, 4-chlorocarbonyl-2-nitrothiophenol,4-chlorocarbonyl-2-aminothiophenol,4-chlorocarbonyl-2-hydroxythiophenol,4-chlorocarbonyl-2-phenylthiothiophenol, 4-sulfo-2-nitrothiophenol,4-sulfo-2-aminothiophenol, 4-sulfo-2-hydroxythiophenol,4-sulfo-2-phenylthiothiophenol, 4-methoxysulfonyl-2-nitrothiophenol,4-methoxysulfonyl-2-aminothiophenol,4-methoxysulfonyl-2-hydroxythiophenol,4-methoxysulfonyl-2-phenylthiothiophenol,4-chlorosulfonyl-2-nitrothiophenol, 4-chlorosulfonyl-2-aminothiophenol,4-chlorosulfonyl-2-hydroxythiophenol,4-chlorosulfonyl-2-phenylthiothiophenol, 4-sulfino-2-nitrothiophenol,4-sulfino-2-aminothiophenol, 4-sulfino-2-hydroxythiophenol,4-sulfino-2-phenylthiothiophenol, 4-methylsulfinyl-2-nitrothiophenol,4-methylsulfinyl-2-aminothiophenol,4-methylsulfinyl-2-hydroxythiophenol,4-methylsulfinyl-2-phenylthiothiophenol, 4-carbamoyl-2-nitrothiophenol,4-carbamoyl-2-aminothiophenol, 4-carbamoyl-2-hydroxythiophenol,4-carbamoyl-2-phenylthiothiophenol, 4-trichloromethyl-2-nitrothiophenol,4-trichloromethyl-2-aminothiophenol,4-trichloromethyl-2-hydroxythiophenol,4-trichloromethyl-2-phenylthiothiophenol, 4-cyano-2-nitrothiophenol,4-cyano-2-aminothiophenol, 4-cyano-2-hydroxythiophenol,4-cyano-2-phenylthiothiophenol, 4-methoxy-2-nitrothiophenol,4-methoxy-2-aminothiophenol, 4-methoxy-2-hydroxythiophenol, and4-methoxy-2-phenylthiothiophenol.

Still another example of the organic sulfur compound represented by thechemical formula (1) is a compound substituted with two or more types ofsubstituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol, 4-acetyl-2-methylthiophenol,4-acetyl-2-carboxythiophenol, 4-acetyl-2-methoxycarbonylthiophenol,4-acetyl-2-formylthiophenol, 4-acetyl-2-chlorocarbonylthiophenol,4-acetyl-2-sulfothiophenol, 4-acetyl-2-methoxysulfonylthiophenol,4-acetyl-2-chlorosulfonylthiophenol, 4-acetyl-2-sulfinothiophenol,4-acetyl-2-methylsulfinylthiophenol, 4-acetyl-2-carbamoylthiophenol,4-acetyl-2-trichloromethylthiophenol, 4-acetyl-2-cyanothiophenol, and4-acetyl-2-methoxythiophenol.

Examples of the organic sulfur compound represented by the chemicalformula (2) include diphenyl disulfide; diphenyl disulfides substitutedwith halogen groups, such as bis(4-fluorophenyl)disulfide,bis(2,5-difluorophenyl)disulfide, bis(2,4,5-trifluorophenyl)disulfide,bis(2,4,5,6-tetrafluorophenyl)disulfide,bis(pentafluorophenyl)disulfide, bis(4-chlorophenyl)disulfide,bis(2,5-dichlorophenyl)disulfide, bis(2,4,5-trichlorophenyl)disulfide,bis(2,4,5,6-tetrachlorophenyl)disulfide,bis(pentachlorophenyl)disulfide, bis(4-bromophenyl)disulfide,bis(2,5-dibromophenyl)disulfide, bis(2,4,5-tribromophenyl)disulfide,bis(2,4,5,6-tetrabromophenyl)disulfide, bis(pentabromophenyl)disulfide,bis(4-iodophenyl)disulfide, bis(2,5-diiodophenyl)disulfide,bis(2,4,5-triiodophenyl)disulfide,bis(2,4,5,6-tetraiodophenyl)disulfide, andbis(pentaiodophenyl)disulfide; diphenyl disulfides substituted withalkyl groups, such as bis(4-methylphenyl)disulfide,bis(2,4,5-trimethylphenyl)disulfide, bis(pentamethylphenyl)disulfide,bis(4-t-butylphenyl)disulfide, bis(2,4,5-tri-t-butylphenyl)disulfide,and bis(penta-t-butylphenyl)disulfide; diphenyl disulfides substitutedwith carboxyl groups, such as bis(4-carboxyphenyl)disulfide,bis(2,4,6-tricarboxyphenyl)disulfide, andbis(pentacarboxyphenyl)disulfide; diphenyl disulfides substituted withalkoxycarbonyl groups, such as bis(4-methoxycarbonylphenyl)disulfide,bis(2,4,6-trimethoxycarbonylphenyl)disulfide, andbis(pentamethoxycarbonylphenyl)disulfide; diphenyl disulfidessubstituted with formyl groups, such as bis(4-formylphenyl)disulfide,bis(2,4,6-triformylphenyl)disulfide, andbis(pentaformylphenyl)disulfide; diphenyl disulfides substituted withacyl groups, such as bis(4-acetylphenyl)disulfide,bis(2,4,6-triacetylphenyl)disulfide, andbis(pentaacetylphenyl)disulfide; diphenyl disulfides substituted withcarbonyl halide groups, such as bis(4-chlorocarbonylphenyl)disulfide,bis(2,4,6-tri(chlorocarbonyl)phenyl)disulfide, andbis(penta(chlorocarbonyl)phenyl)disulfide; diphenyl disulfidessubstituted with sulfo groups, such as bis(4-sulfophenyl)disulfide,bis(2,4,6-trisulfophenyl)disulfide, and bis(pentasulfophenyl)disulfide;diphenyl disulfides substituted with alkoxysulfonyl groups, such asbis(4-methoxysulfonylphenyl)disulfide,bis(2,4,6-trimethoxysulfonylphenyl)disulfide, andbis(pentamethoxysulfonylphenyl)disulfide; diphenyl disulfidessubstituted with sulfonyl halide groups, such asbis(4-chlorosulfonylphenyl)disulfide,bis(2,4,6-tri(chlorosulfonyl)phenyl)disulfide, andbis(penta(chlorosulfonyl)phenyl)disulfide; diphenyl disulfidessubstituted with sulfino groups, such as bis(4-sulfinophenyl)disulfide,bis(2,4,6-trisulfinophenyl)disulfide, andbis(pentasulfinophenyl)disulfide; diphenyl disulfides substituted withalkylsulfinyl groups, such as bis(4-methylsulfinylphenyl)disulfide,bis(2,4,6-tri(methylsulfinyl)phenyl)disulfide, andbis(penta(methylsulfinyl)phenyl)disulfide; diphenyl disulfidessubstituted with carbamoyl groups, such asbis(4-carbamoylphenyl)disulfide, bis(2,4,6-tricarbamoylphenyl)disulfide,and bis(pentacarbamoylphenyl)disulfide; diphenyl disulfides substitutedwith alkyl halide groups, such as bis(4-trichloromethylphenyl)disulfide,bis(2,4,6-tri(trichloromethyl)phenyl)disulfide, andbis(penta(trichloromethyl)phenyl)disulfide; diphenyl disulfidessubstituted with cyano groups, such as bis(4-cyanophenyl)disulfide,bis(2,4,6-tricyanophenyl)disulfide, and bis(pentacyanophenyl)disulfide;and diphenyl disulfides substituted with alkoxy groups, such asbis(4-methoxyphenyl)disulfide, bis(2,4,6-trimethoxyphenyl)disulfide, andbis(pentamethoxyphenyl)disulfide. Each of these diphenyl disulfides issubstituted with one type of substituent.

Another example of the organic sulfur compound represented by thechemical formula (2) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include bis(4-chloro-2-nitrophenyl)disulfide,bis(4-chloro-2-aminophenyl)disulfide,bis(4-chloro-2-hydroxyphenyl)disulfide,bis(4-chloro-2-phenylthiophenyl)disulfide,bis(4-methyl-2-nitrophenyl)disulfide,bis(4-methyl-2-aminophenyl)disulfide,bis(4-methyl-2-hydroxyphenyl)disulfide,bis(4-methyl-2-phenylthiophenyl)disulfide,bis(4-carboxy-2-nitrophenyl)disulfide,bis(4-carboxy-2-aminophenyl)disulfide,bis(4-carboxy-2-hydroxyphenyl)disulfide,bis(4-carboxy-2-phenylthiophenyl)disulfide,bis(4-methoxycarbonyl-2-nitrophenyl)disulfide,bis(4-methoxycarbonyl-2-aminophenyl)disulfide,bis(4-methoxycarbonyl-2-hydroxyphenyl)disulfide,bis(4-methoxycarbonyl-2-phenylthiophenyl)disulfide,bis(4-formyl-2-nitrophenyl)disulfide,bis(4-formyl-2-aminophenyl)disulfide,bis(4-formyl-2-hydroxyphenyl)disulfide,bis(4-formyl-2-phenylthiophenyl)disulfide,bis(4-acetyl-2-nitrophenyl)disulfide,bis(4-acetyl-2-aminophenyl)disulfide,bis(4-acetyl-2-hydroxyphenyl)disulfide,bis(4-acetyl-2-phenylthiophenyl)disulfide,bis(4-chlorocarbonyl-2-nitrophenyl)disulfide,bis(4-chlorocarbonyl-2-aminophenyl)disulfide,bis(4-chlorocarbonyl-2-hydroxyphenyl)disulfide,bis(4-chlorocarbonyl-2-phenylthiophenyl)disulfide,bis(4-sulfo-2-nitrophenyl)disulfide,bis(4-sulfo-2-aminophenyl)disulfide,bis(4-sulfo-2-hydroxyphenyl)disulfide,bis(4-sulfo-2-phenylthiophenyl)disulfide,bis(4-methoxysulfonyl-2-nitrophenyl)disulfide,bis(4-methoxysulfonyl-2-aminophenyl)disulfide,bis(4-methoxysulfonyl-2-hydroxyphenyl)disulfide,bis(4-methoxysulfonyl-2-phenylthiophenyl)disulfide,bis(4-chlorosulfonyl-2-nitrophenyl)disulfide,bis(4-chlorosulfonyl-2-aminophenyl)disulfide,bis(4-chlorosulfonyl-2-hydroxyphenyl)disulfide,bis(4-chlorosulfonyl-2-phenylthiophenyl)disulfide,bis(4-sulfino-2-nitrophenyl)disulfide,bis(4-sulfino-2-aminophenyl)disulfide,bis(4-sulfino-2-hydroxyphenyl)disulfide,bis(4-sulfino-2-phenylthiophenyl)disulfide,bis(4-methylsulfinyl-2-nitrophenyl)disulfide,bis(4-methylsulfinyl-2-aminophenyl)disulfide,bis(4-methylsulfinyl-2-hydroxyphenyl)disulfide,bis(4-methylsulfinyl-2-phenylthiophenyl)disulfide,bis(4-carbamoyl-2-nitrophenyl)disulfide,bis(4-carbamoyl-2-aminophenyl)disulfide,bis(4-carbamoyl-2-hydroxyphenyl)disulfide,bis(4-carbamoyl-2-phenylthiophenyl)disulfide,bis(4-trichloromethyl-2-nitrophenyl)disulfide,bis(4-trichloromethyl-2-aminophenyl)disulfide,bis(4-trichloromethyl-2-hydroxyphenyl)disulfide,bis(4-trichloromethyl-2-phenylthiophenyl)disulfide,bis(4-cyano-2-nitrophenyl)disulfide,bis(4-cyano-2-aminophenyl)disulfide,bis(4-cyano-2-hydroxyphenyl)disulfide,bis(4-cyano-2-phenylthiophenyl)disulfide,bis(4-methoxy-2-nitrophenyl)disulfide,bis(4-methoxy-2-aminophenyl)disulfide,bis(4-methoxy-2-hydroxyphenyl)disulfide, andbis(4-methoxy-2-phenylthiophenyl)disulfide.

Still another example of the organic sulfur compound represented by thechemical formula (2) is a compound substituted with two or more types ofsubstituents. Specific examples of the compound includebis(4-acetyl-2-chlorophenyl)disulfide,bis(4-acetyl-2-methylphenyl)disulfide,bis(4-acetyl-2-carboxyphenyl)disulfide,bis(4-acetyl-2-methoxycarbonylphenyl)disulfide,bis(4-acetyl-2-formylphenyl)disulfide,bis(4-acetyl-2-chlorocarbonylphenyl)disulfide,bis(4-acetyl-2-sulfophenyl)disulfide,bis(4-acetyl-2-methoxysulfonylphenyl)disulfide,bis(4-acetyl-2-chlorosulfonylphenyl)disulfide,bis(4-acetyl-2-sulfinophenyl)disulfide,bis(4-acetyl-2-methylsulfinylphenyl)disulfide,bis(4-acetyl-2-carbamoylphenyl)disulfide,bis(4-acetyl-2-trichloromethylphenyl)disulfide,bis(4-acetyl-2-cyanophenyl)disulfide, andbis(4-acetyl-2-methoxyphenyl)disulfide.

Examples of the organic sulfur compound represented by the chemicalformula (3) include thiophenol sodium salt; thiophenol sodium saltssubstituted with halogen groups, such as 4-fluorothiophenol sodium salt,2,5-difluorothiophenol sodium salt, 2,4,5-trifluorothiophenol sodiumsalt, 2,4,5,6-tetrafluorothiophenol sodium salt, pentafluorothiophenolsodium salt, 4-chlorothiophenol sodium salt, 2,5-dichlorothiophenolsodium salt, 2,4,5-trichlorothiophenol sodium salt,2,4,5,6-tetrachlorothiophenol sodium salt, pentachlorothiophenol sodiumsalt, 4-bromothiophenol sodium salt, 2,5-dibromothiophenol sodium salt,2,4,5-tribromothiophenol sodium salt, 2,4,5,6-tetrabromothiophenolsodium salt, pentabromothiophenol sodium salt, 4-iodothiophenol sodiumsalt, 2,5-diiodothiophenol sodium salt, 2,4,5-triiodothiophenol sodiumsalt, 2,4,5,6-tetraiodothiophenol sodium salt, and pentaiodothiophenolsodium salt; thiophenol sodium salts substituted with alkyl groups, suchas 4-methylthiophenol sodium salt, 2,4,5-trimethylthiophenol sodiumsalt, pentamethylthiophenol sodium salt, 4-t-butylthiophenol sodiumsalt, 2,4,5-tri-t-butylthiophenol sodium salt, andpenta(t-butyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with carboxyl groups, such as 4-carboxythiophenol sodiumsalt, 2,4,6-tricarboxythiophenol sodium salt, and pentacarboxythiophenolsodium salt; thiophenol sodium salts substituted with alkoxycarbonylgroups, such as 4-methoxycarbonylthiophenol sodium salt,2,4,6-trimethoxycarbonylthiophenol sodium salt, andpentamethoxycarbonylthiophenol sodium salt; thiophenol sodium saltssubstituted with formyl groups, such as 4-formylthiophenol sodium salt,2,4,6-triformylthiophenol sodium salt, and pentaformylthiophenol sodiumsalt; thiophenol sodium salts substituted with acyl groups, such as4-acetylthiophenol sodium salt, 2,4,6-triacetylthiophenol sodium salt,and pentaacetylthiophenol sodium salt; thiophenol sodium saltssubstituted with carbonyl halide groups, such as4-chlorocarbonylthiophenol sodium salt,2,4,6-tri(chlorocarbonyl)thiophenol sodium salt, andpenta(chlorocarbonyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with sulfo groups, such as 4-sulfothiophenol sodium salt,2,4,6-trisulfothiophenol sodium salt, and pentasulfothiophenol sodiumsalt; thiophenol sodium salts substituted with alkoxysulfonyl groups,such as 4-methoxysulfonylthiophenol sodium salt,2,4,6-trimethoxysulfonylthiophenol sodium salt, andpentamethoxysulfonylthiophenol sodium salt; thiophenol sodium saltssubstituted with sulfonyl halide groups, such as4-chlorosulfonylthiophenol sodium salt,2,4,6-tri(chlorosulfonyl)thiophenol sodium salt, andpenta(chlorosulfonyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with sulfino groups, such as 4-sulfinothiophenol sodiumsalt, 2,4,6-trisulfinothiophenol sodium salt, and pentasulfinothiophenolsodium salt; thiophenol sodium salts substituted with alkylsulfinylgroups, such as 4-methylsulfinylthiophenol sodium salt,2,4,6-tri(methylsulfinyl)thiophenol sodium salt, and penta(methylsulfinyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with carbamoyl groups, such as 4-carbamoylthiophenol sodiumsalt, 2,4,6-tricarbamoylthiophenol sodium salt, andpentacarbamoylthiophenol sodium salt; thiophenol sodium saltssubstituted with alkyl halide groups, such as4-trichloromethylthiophenol sodium salt,2,4,6-tri(trichloromethyl)thiophenol sodium salt, andpenta(trichloromethyl)thiophenol sodium salt; thiophenol sodium saltssubstituted with cyano groups, such as 4-cyanothiophenol sodium salt,2,4,6-tricyanothiophenol sodium salt, and pentacyanothiophenol sodiumsalt; and thiophenol sodium salts substituted with alkoxy groups, suchas 4-methoxythiophenol sodium salt, 2,4,6-trimethoxythiophenol sodiumsalt, and pentamethoxythiophenol sodium salt. Each of these thiophenolsodium salts is substituted with one type of substituent.

Another example of the organic sulfur compound represented by thechemical formula (3) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol sodium salt,4-chloro-2-aminothiophenol sodium salt, 4-chloro-2-hydroxythiophenolsodium salt, 4-chloro-2-phenylthiothiophenol sodium salt,4-methyl-2-nitrothiophenol sodium salt, 4-methyl-2-aminothiophenolsodium salt, 4-methyl-2-hydroxythiophenol sodium salt,4-methyl-2-phenylthiothiophenol sodium salt, 4-carboxy-2-nitrothiophenolsodium salt, 4-carboxy-2-aminothiophenol sodium salt,4-carboxy-2-hydroxythiophenol sodium salt,4-carboxy-2-phenylthiothiophenol sodium salt,4-methoxycarbonyl-2-nitrothiophenol sodium salt,4-methoxycarbonyl-2-aminothiophenol sodium salt,4-methoxycarbonyl-2-hydroxythiophenol sodium salt,4-methoxycarbonyl-2-phenylthiothiophenol sodium salt,4-formyl-2-nitrothiophenol sodium salt, 4-formyl-2-aminothiophenolsodium salt, 4-formyl-2-hydroxythiophenol sodium salt,4-formyl-2-phenylthiothiophenol sodium salt, 4-acetyl-2-nitrothiophenolsodium salt, 4-acetyl-2-aminothiophenol sodium salt,4-acetyl-2-hydroxythiophenol sodium salt,4-acetyl-2-phenylthiothiophenol sodium salt,4-chlorocarbonyl-2-nitrothiophenol sodium salt,4-chlorocarbonyl-2-aminothiophenol sodium salt,4-chlorocarbonyl-2-hydroxythiophenol sodium salt,4-chlorocarbonyl-2-phenylthiothiophenol sodium salt,4-sulfo-2-nitrothiophenol sodium salt, 4-sulfo-2-aminothiophenol sodiumsalt, 4-sulfo-2-hydroxythiophenol sodium salt,4-sulfo-2-phenylthiothiophenol sodium salt,4-methoxysulfonyl-2-nitrothiophenol sodium salt,4-methoxysulfonyl-2-aminothiophenol sodium salt,4-methoxysulfonyl-2-hydroxythiophenol sodium salt,4-methoxysulfonyl-2-phenylthiothiophenol sodium salt,4-chlorosulfonyl-2-nitrothiophenol sodium salt,4-chlorosulfonyl-2-aminothiophenol sodium salt,4-chlorosulfonyl-2-hydroxythiophenol sodium salt,4-chlorosulfonyl-2-phenylthiothiophenol sodium salt,4-sulfino-2-nitrothiophenol sodium salt, 4-sulfino-2-aminothiophenolsodium salt, 4-sulfino-2-hydroxythiophenol sodium salt,4-sulfino-2-phenylthiothiophenol sodium salt,4-methylsulfinyl-2-nitrothiophenol sodium salt,4-methylsulfinyl-2-aminothiophenol sodium salt,4-methylsulfinyl-2-hydroxythiophenol sodium salt,4-methylsulfinyl-2-phenylthiothiophenol sodium salt,4-carbamoyl-2-nitrothiophenol sodium salt, 4-carbamoyl-2-aminothiophenolsodium salt, 4-carbamoyl-2-hydroxythiophenol sodium salt,4-carbamoyl-2-phenylthiothiophenol sodium salt,4-trichloromethyl-2-nitrothiophenol sodium salt,4-trichloromethyl-2-aminothiophenol sodium salt,4-trichloromethyl-2-hydroxythiophenol sodium salt,4-trichloromethyl-2-phenylthiothiophenol sodium salt,4-cyano-2-nitrothiophenol sodium salt, 4-cyano-2-aminothiophenol sodiumsalt, 4-cyano-2-hydroxythiophenol sodium salt,4-cyano-2-phenylthiothiophenol sodium salt, 4-methoxy-2-nitrothiophenolsodium salt, 4-methoxy-2-aminothiophenol sodium salt,4-methoxy-2-hydroxythiophenol sodium salt, and4-methoxy-2-phenylthiothiophenol sodium salt.

Still another example of the organic sulfur compound represented by thechemical formula (3) is a compound substituted with two or more types ofsubstituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol sodium salt, 4-acetyl-2-methylthiophenolsodium salt, 4-acetyl-2-carboxythiophenol sodium salt,4-acetyl-2-methoxycarbonylthiophenol sodium salt,4-acetyl-2-formylthiophenol sodium salt,4-acetyl-2-chlorocarbonylthiophenol sodium salt,4-acetyl-2-sulfothiophenol sodium salt,4-acetyl-2-methoxysulfonylthiophenol sodium salt,4-acetyl-2-chlorosulfonylthiophenol sodium salt,4-acetyl-2-sulfinothiophenol sodium salt,4-acetyl-2-methylsulfinylthiophenol sodium salt,4-acetyl-2-carbamoylthiophenol sodium salt,4-acetyl-2-trichloromethylthiophenol sodium salt,4-acetyl-2-cyanothiophenol sodium salt, and 4-acetyl-2-methoxythiophenolsodium salt. Examples of the monovalent metal represented by M1 in thechemical formula (3) include sodium, lithium, potassium, copper (I), andsilver (I).

Examples of the organic sulfur compound represented by the chemicalformula (4) include thiophenol zinc salt; thiophenol zinc saltssubstituted with halogen groups, such as 4-fluorothiophenol zinc salt,2,5-difluorothiophenol zinc salt, 2,4,5-trifluorothiophenol zinc salt,2,4,5,6-tetrafluorothiophenol zinc salt, pentafluorothiophenol zincsalt, 4-chlorothiophenol zinc salt, 2,5-dichlorothiophenol zinc salt,2,4,5-trichlorothiophenol zinc salt, 2,4,5,6-tetrachlorothiophenol zincsalt, pentachlorothiophenol zinc salt, 4-bromothiophenol zinc salt,2,5-dibromothiophenol zinc salt, 2,4,5-tribromothiophenol zinc salt,2,4,5,6-tetrabromothiophenol zinc salt, pentabromothiophenol zinc salt,4-iodothiophenol zinc salt, 2,5-diiodothiophenol zinc salt,2,4,5-triiodothiophenol zinc salt, 2,4,5,6-tetraiodothiophenol zincsalt, and pentaiodothiophenol zinc salt; thiophenol zinc saltssubstituted with alkyl groups, such as 4-methylthiophenol zinc salt,2,4,5-trimethylthiophenol zinc salt, pentamethylthiophenol zinc salt,4-t-butylthiophenol zinc salt, 2,4,5-tri-t-butylthiophenol zinc salt,and penta-t-butylthiophenol zinc salt; thiophenol zinc salts substitutedwith carboxyl groups, such as 4-carboxythiophenol zinc salt,2,4,6-tricarboxythiophenol zinc salt, and pentacarboxythiophenol zincsalt; thiophenol zinc salts substituted with alkoxycarbonyl groups, suchas 4-methoxycarbonylthiophenol zinc salt,2,4,6-trimethoxycarbonylthiophenol zinc salt, andpentamethoxycarbonylthiophenol zinc salt; thiophenol zinc saltssubstituted with formyl groups, such as 4-formylthiophenol zinc salt,2,4,6-triformylthiophenol zinc salt, and pentaformylthiophenol zincsalt; thiophenol zinc salts substituted with acyl groups, such as4-acetylthiophenol zinc salt, 2,4,6-triacetylthiophenol zinc salt, andpentaacetylthiophenol zinc salt; thiophenol zinc salts substituted withcarbonyl halide groups, such as 4-chlorocarbonylthiophenol zinc salt,2,4,6-tri(chlorocarbonyl)thiophenol zinc salt, andpenta(chlorocarbonyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with sulfo groups, such as 4-sulfothiophenol zinc salt,2,4,6-trisulfothiophenol zinc salt, and pentasulfothiophenol zinc salt;thiophenol zinc salts substituted with alkoxysulfonyl groups, such as4-methoxysulfonylthiophenol zinc salt,2,4,6-trimethoxysulfonylthiophenol zinc salt, andpentamethoxysulfonylthiophenol zinc salt; thiophenol zinc saltssubstituted with sulfonyl halide groups, such as4-chlorosulfonylthiophenol zinc salt,2,4,6-tri(chlorosulfonyl)thiophenol zinc salt, andpenta(chlorosulfonyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with sulfino groups, such as 4-sulfinothiophenol zinc salt,2,4,6-trisulfinothiophenol zinc salt, and pentasulfinothiophenol zincsalt; thiophenol zinc salts substituted with alkylsulfinyl groups, suchas 4-methylsulfinylthiophenol zinc salt,2,4,6-tri(methylsulfinyl)thiophenol zinc salt, andpenta(methylsulfinyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with carbamoyl groups, such as 4-carbamoylthiophenol zincsalt, 2,4,6-tricarbamoylthiophenol zinc salt, andpentacarbamoylthiophenol zinc salt; thiophenol zinc salts substitutedwith alkyl halide groups, such as 4-trichloromethylthiophenol zinc salt,2,4,6-tri(trichloromethyl)thiophenol zinc salt, andpenta(trichloromethyl)thiophenol zinc salt; thiophenol zinc saltssubstituted with cyano groups, such as 4-cyanothiophenol zinc salt,2,4,6-tricyanothiophenol zinc salt, and pentacyanothiophenol zinc salt;and thiophenol zinc salts substituted with alkoxy groups, such as4-methoxythiophenol zinc salt, 2,4,6-trimethoxythiophenol zinc salt, andpentamethoxythiophenol zinc salt. Each of these thiophenol zinc salts issubstituted with one type of substituent.

Another example of the organic sulfur compound represented by thechemical formula (4) is a compound substituted with at least one type ofthe above substituents and another substituent. Examples of the othersubstituent include a nitro group (—NO₂), an amino group (—NH₂), ahydroxyl group (—OH), and a phenylthio group (—SPh). Specific examplesof the compound include 4-chloro-2-nitrothiophenol zinc salt,4-chloro-2-aminothiophenol zinc salt, 4-chloro-2-hydroxythiophenol zincsalt, 4-chloro-2-phenylthiothiophenol zinc salt,4-methyl-2-nitrothiophenol zinc salt, 4-methyl-2-aminothiophenol zincsalt, 4-methyl-2-hydroxythiophenol zinc salt,4-methyl-2-phenylthiothiophenol zinc salt, 4-carboxy-2-nitrothiophenolzinc salt, 4-carboxy-2-aminothiophenol zinc salt,4-carboxy-2-hydroxythiophenol zinc salt,4-carboxy-2-phenylthiothiophenol zinc salt,4-methoxycarbonyl-2-nitrothiophenol zinc salt,4-methoxycarbonyl-2-aminothiophenol zinc salt,4-methoxycarbonyl-2-hydroxythiophenol zinc salt,4-methoxycarbonyl-2-phenylthiothiophenol zinc salt,4-formyl-2-nitrothiophenol zinc salt, 4-formyl-2-aminothiophenol zincsalt, 4-formyl-2-hydroxythiophenol zinc salt,4-formyl-2-phenylthiothiophenol zinc salt, 4-acetyl-2-nitrothiophenolzinc salt, 4-acetyl-2-aminothiophenol zinc salt,4-acetyl-2-hydroxythiophenol zinc salt, 4-acetyl-2-phenylthiothiophenolzinc salt, 4-chlorocarbonyl-2-nitrothiophenol zinc salt,4-chlorocarbonyl-2-aminothiophenol zinc salt,4-chlorocarbonyl-2-hydroxythiophenol zinc salt,4-chlorocarbonyl-2-phenylthiothiophenol zinc salt,4-sulfo-2-nitrothiophenol zinc salt, 4-sulfo-2-aminothiophenol zincsalt, 4-sulfo-2-hydroxythiophenol zinc salt,4-sulfo-2-phenylthiothiophenol zinc salt,4-methoxysulfonyl-2-nitrothiophenol zinc salt,4-methoxysulfonyl-2-aminothiophenol zinc salt,4-methoxysulfonyl-2-hydroxythiophenol zinc salt,4-methoxysulfonyl-2-phenylthiothiophenol zinc salt,4-chlorosulfonyl-2-nitrothiophenol zinc salt,4-chlorosulfonyl-2-aminothiophenol zinc salt,4-chlorosulfonyl-2-hydroxythiophenol zinc salt,4-chlorosulfonyl-2-phenylthiothiophenol zinc salt,4-sulfino-2-nitrothiophenol zinc salt, 4-sulfino-2-aminothiophenol zincsalt, 4-sulfino-2-hydroxythiophenol zinc salt,4-sulfino-2-phenylthiothiophenol zinc salt,4-methylsulfinyl-2-nitrothiophenol zinc salt,4-methylsulfinyl-2-aminothiophenol zinc salt,4-methylsulfinyl-2-hydroxythiophenol zinc salt,4-methylsulfinyl-2-phenylthiothiophenol zinc salt,4-carbamoyl-2-nitrothiophenol zinc salt, 4-carbamoyl-2-aminothiophenolzinc salt, 4-carbamoyl-2-hydroxythiophenol zinc salt,4-carbamoyl-2-phenylthiothiophenol zinc salt,4-trichloromethyl-2-nitrothiophenol zinc salt,4-trichloromethyl-2-aminothiophenol zinc salt,4-trichloromethyl-2-hydroxythiophenol zinc salt,4-trichloromethyl-2-phenylthiothiophenol zinc salt,4-cyano-2-nitrothiophenol zinc salt, 4-cyano-2-aminothiophenol zincsalt, 4-cyano-2-hydroxythiophenol zinc salt,4-cyano-2-phenylthiothiophenol zinc salt, 4-methoxy-2-nitrothiophenolzinc salt, 4-methoxy-2-aminothiophenol zinc salt,4-methoxy-2-hydroxythiophenol zinc salt, and4-methoxy-2-phenylthiothiophenol zinc salt.

Still another example of the organic sulfur compound represented by thechemical formula (4) is a compound substituted with two or more types ofsubstituents. Specific examples of the compound include4-acetyl-2-chlorothiophenol zinc salt, 4-acetyl-2-methylthiophenol zincsalt, 4-acetyl-2-carboxythiophenol zinc salt,4-acetyl-2-methoxycarbonylthiophenol zinc salt,4-acetyl-2-formylthiophenol zinc salt,4-acetyl-2-chlorocarbonylthiophenol zinc salt,4-acetyl-2-sulfothiophenol zinc salt,4-acetyl-2-methoxysulfonylthiophenol zinc salt,4-acetyl-2-chlorosulfonylthiophenol zinc salt,4-acetyl-2-sulfinothiophenol zinc salt,4-acetyl-2-methylsulfinylthiophenol zinc salt,4-acetyl-2-carbamoylthiophenol zinc salt,4-acetyl-2-trichloromethylthiophenol zinc salt,4-acetyl-2-cyanothiophenol zinc salt, and 4-acetyl-2-methoxythiophenolzinc salt. Examples of the bivalent metal represented by M2 in thechemical formula (4) include zinc, magnesium, calcium, strontium,barium, titanium (II), manganese (II), iron (II), cobalt (II), nickel(II), zirconium (II), and tin (II).

Examples of thionaphthols include 2-thionaphthol, 1-thionaphthol,2-chloro-1-thionaphthol, 2-bromo-1-thionaphthol,2-fluoro-1-thionaphthol, 2-cyano-1-thionaphthol,2-acetyl-1-thionaphthol, 1-chloro-2-thionaphthol,1-bromo-2-thionaphthol, 1-fluoro-2-thionaphthol, 1-cyano-2-thionaphthol,1-acetyl-2-thionaphthol, and metal salts thereof. 1-thionaphthol,2-thionaphthol, and zinc salts thereof are preferred.

Examples of sulfenamide type organic sulfur compounds includeN-cyclohexyl-2-benzothiazole sulfenamide,N-oxydiethylene-2-benzothiazole sulfenamide, andN-t-butyl-2-benzothiazole sulfenamide. Examples of thiuram type organicsulfur compounds include tetramethylthiuram monosulfide,tetramethylthiuram disulfide, tetraethylthiuram disulfide,tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide.Examples of dithiocarbamates include zinc dimethyldithiocarbamate, zincdiethyldithiocarbamate, zinc dibutyldithiocarbamate, zincethylphenyldithiocarbamate, sodium dimethyldithiocarbamate, sodiumdiethyldithiocarbamate, copper (II) dimethyldithiocarbamate, iron (III)dimethyldithiocarbamate, selenium diethyldithiocarbamate, and telluriumdiethyldithiocarbamate. Examples of thiazole type organic sulfurcompounds include 2-mercaptobenzothiazole (MBT); dibenzothiazyldisulfide (MBTS); a sodium salt, a zinc salt, a copper salt, or acyclohexylamine salt of 2-mercaptobenzothiazole;2-(2,4-dinitrophenyl)mercaptobenzothiazole; and2-(2,6-diethyl-4-morpholinothio)benzothiazole

In light of resilience performance, the amount of the organic sulfurcompound (e) is preferably equal to or greater than 0.05 parts by weightand particularly preferably equal to or greater than 0.1 parts byweight, per 100 parts by weight of the base rubber. In light ofresilience performance, the amount is preferably equal to or less than5.0 parts by weight and particularly preferably equal to or less than2.0 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the core 4. Examples of suitable fillers include zincoxide, barium sulfate, calcium carbonate, and magnesium carbonate. Theamount of the filler is determined as appropriate so that the intendedspecific gravity of the core 4 is accomplished. A particularlypreferable filler is zinc oxide. Zinc oxide serves not only as aspecific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer,a dispersant, sulfur, a vulcanization accelerator, and the like areadded to the rubber composition of the core 4. Crosslinked rubber powderor synthetic resin powder may also be dispersed in the rubbercomposition.

In the core 4, the difference (Hs−H(0)) between the surface hardness Hsand the central hardness H(0) is preferably equal to or greater than 15.The difference is great. In other words, the core 4 has anouter-hard/inner-soft structure. When the core 4 is hit with a middleiron, the recoil (torsional return) is great, and thus spin issuppressed. The core 4 contributes to the flight performance of the golfball 2. In light of flight performance, the difference (Hs−H(0)) is morepreferably equal to or greater than 20 and particularly preferably equalto or greater than 25. From the standpoint that the core 4 can easily beformed, the difference (Hs−H(0)) is preferably equal to or less than 50.

The hardness H(0) at the central point of the core 4 is preferably equalto or greater than 40.0 but equal to or less than 70.0. The golf ball 2having a hardness H(0) of 40.0 or greater has excellent resilienceperformance. In this respect, the hardness H(0) is more preferably equalto or greater than 45.0 and particularly preferably equal to or greaterthan 50.0. The core 4 having a hardness H(0) of 70.0 or less can achievean outer-hard/inner-soft structure. In the golf ball 2 that includes thecore 4, spin can be suppressed. In this respect, the hardness H(0) ismore preferably equal to or less than 68.0 and particularly preferablyequal to or less than 66.0.

The hardness Hs at the surface of the core 4 is preferably equal to orgreater than 78.0 but equal to or less than 95.0. In the core 4 having ahardness Hs of 78.0 or greater, an outer-hard/inner-soft structure canbe achieved. In the golf ball 2 that includes the core 4, spin can besuppressed. In this respect, the hardness Hs is more preferably equal toor greater than 80.0 and particularly preferably equal to or greaterthan 82.0. The golf ball 2 having a hardness Hs of 95.0 or less hasexcellent durability. In this respect, the hardness Hs is morepreferably equal to or less than 93.0 and particularly preferably equalto or less than 90.0.

The core 4 preferably has a diameter of 38.0 mm or greater but 42.0 mmor less. The core 4 having a diameter of 38.0 mm or greater can achieveexcellent resilience performance of the golf ball 2. In this respect,the diameter is more preferably equal to or greater than 38.5 mm andparticularly preferably equal to or greater than 39.0 mm. In the golfball 2 that includes the core 4 having a diameter of 42.0 mm or less,the inner cover 8 and the outer cover 10 can have sufficientthicknesses. The golf ball 2 that includes the inner cover 8 and theouter cover 10 which have large thicknesses has excellent durability. Inthis respect, the diameter is more preferably equal to or less than 41.0mm and particularly preferably equal to or less than 40.0 mm.

In light of feel at impact, the core 4 has an amount of compressivedeformation Dc of preferably 3.0 mm or greater and particularlypreferably 3.3 mm or greater. In light of resilience performance, theamount of compressive deformation Dc is preferably equal to or less than4.6 mm and particularly preferably equal to or less than 4.3 mm.

For the inner cover 8, a resin composition is suitably used. Examples ofthe base polymer of the resin composition include ionomer resins,styrene block-containing thermoplastic elastomers, thermoplasticpolyester elastomers, thermoplastic polyamide elastomers, andthermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. The golf ball2 that includes the inner cover 8 including an ionomer resin hasexcellent resilience performance. An ionomer resin and another resin maybe used in combination for the inner cover 8. In this case, theprincipal component of the base polymer is preferably the ionomer resin.Specifically, the proportion of the ionomer resin to the entire basepolymer is preferably equal to or greater than 50% by weight, morepreferably equal to or greater than 60% by weight, and particularlypreferably equal to or greater than 70% by weight.

Examples of preferable ionomer resins include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. A preferable binary copolymer includes 80% by weight orgreater but 90% by weight or less of an α-olefin, and 10% by weight orgreater but 20% by weight or less of an α,β-unsaturated carboxylic acid.The binary copolymer has excellent resilience performance. Examples ofother preferable ionomer resins include ternary copolymers formed with:an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbonatoms. A preferable ternary copolymer includes 70% by weight or greaterbut 85% by weight or less of an α-olefin, 5% by weight or greater but30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% byweight or greater but 25% by weight or less of an α,β-unsaturatedcarboxylate ester. The ternary copolymer has excellent resilienceperformance. For the binary copolymers and the ternary copolymers,preferable α-olefins are ethylene and propylene, while preferableα,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid.A particularly preferable ionomer resin is a copolymer formed withethylene and acrylic acid or methacrylic acid.

In the binary copolymers and the ternary copolymers, some of thecarboxyl groups are neutralized with metal ions. Examples of metal ionsfor use in neutralization include sodium ion, potassium ion, lithiumion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymiumion. The neutralization may be carried out with two or more types ofmetal ions. Particularly suitable metal ions in light of resilienceperformance and durability of the golf ball 2 are sodium ion, zinc ion,lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”,“Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan1856”, “Himilan 1855”, “Himilan AM7311”, “Himilan AM7315”, “HimilanAM7317”, “Himilan AM7318”, “Himilan AM7329”, “Himilan AM7337”, “HimilanMK7320”, and “Himilan MK7329”, manufactured by Du Pont-MITSUIPOLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”,“Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”,“Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufacturedby E. I. du Pont de Nemours and Company; and trade names “IOTEK 7010”,“IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK8030”, manufactured by ExxonMobil Chemical Company.

Two or more ionomer resins may be used in combination for the innercover 8. An ionomer resin neutralized with a monovalent metal ion, andan ionomer resin neutralized with a bivalent metal ion may be used incombination.

A preferable resin that can be used in combination with an ionomer resinis a styrene block-containing thermoplastic elastomer. The styreneblock-containing thermoplastic elastomer has excellent compatibilitywith ionomer resins. A resin composition including the styreneblock-containing thermoplastic elastomer has excellent fluidity.

The styrene block-containing thermoplastic elastomer includes apolystyrene block as a hard segment, and a soft segment. A typical softsegment is a diene block. Examples of compounds for the diene blockinclude butadiene, isoprene, 1,3-pentadiene, and2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two ormore compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers includestyrene-butadiene-styrene block copolymers (SBS),styrene-isoprene-styrene block copolymers (SIS),styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenatedSBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenatedSBS include styrene-ethylene-butylene-styrene block copolymers (SEBS).Examples of hydrogenated SIS include styrene-ethylene-propylene-styreneblock copolymers (SEPS). Examples of hydrogenated SIBS includestyrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 2, the content ofthe styrene component in the styrene block-containing thermoplasticelastomer is preferably equal to or greater than 10% by weight, morepreferably equal to or greater than 12% by weight, and particularlypreferably equal to or greater than 15% by weight. In light of feel atimpact of the golf ball 2, the content is preferably equal to or lessthan 50% by weight, more preferably equal to or less than 47% by weight,and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplasticelastomers include an alloy of an olefin and one or more membersselected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, SEEPS,and hydrogenated products thereof. The olefin component in the alloy ispresumed to contribute to improvement of compatibility with ionomerresins. Use of this alloy improves the resilience performance of thegolf ball 2. An olefin having 2 to 10 carbon atoms is preferably used.Examples of suitable olefins include ethylene, propylene, butene, andpentene. Ethylene and propylene are particularly preferred.

Specific examples of polymer alloys include trade names “RabalonT3221C”,“RabalonT3339C”, “RabalonSJ4400N”, “Rabalon SJ5400N”, “Rabalon SJ6400N”,“Rabalon SJ7400N”, “Rabalon SJ8400N”, “Rabalon SJ9400N”, and “RabalonSR04”, manufactured by Mitsubishi Chemical Corporation. Other specificexamples of styrene block-containing thermoplastic elastomers includetrade name “Epofriend A1010” manufactured by Daicel Chemical Industries,Ltd., and trade name “Septon HG-252” manufactured by Kuraray Co., Ltd.

According to need, a coloring agent such as titanium dioxide and afluorescent pigment, a filler such as barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial, a fluorescent brightener, and the like are included in theresin composition of the inner cover 8 in an adequate amount. The innercover 8 may include powder of a metal with a high specific gravity.

From the standpoint that an outer-hard/inner-soft structure can beachieved in a sphere 16 consisting of the core 4 and the inner cover 8,the inner cover 8 has a hardness Hi of preferably 80 or greater, morepreferably 83 or greater, and particularly preferably 85 or greater. Inlight of feel at impact of the golf ball 2, the hardness Hi ispreferably equal to or less than 95 and particularly preferably equal toor less than 90. The hardness Hi is measured with a JIS-C type hardnessscale mounted to an automated rubber hardness measurement machine (tradename “P1”, manufactured by Kobunshi Keiki Co., Ltd.). For themeasurement, a slab that is formed by hot press and that has a thicknessof about 2 mm is used. A slab kept at 23° C. for two weeks is used forthe measurement. At the measurement, three slabs are stacked. A slabformed from the same resin composition as the resin composition of theinner cover 8 is used.

From the standpoint that an outer-hard/inner-soft structure is achievedin the sphere 16 and spin of the golf ball 2 is suppressed, the hardnessHi of the inner cover 8 is preferably greater than the surface hardnessHs of the core 4. In light of suppression of spin, the difference(Hi−Hs) between the hardness Hi and the hardness Hs is preferably equalto or greater than 1 and particularly preferably equal to or greaterthan 2. The difference (Hi−Hs) is preferably equal to or less than 5. Inthe sphere 16 in which the difference (Hi−Hs) is equal to or less than5, the hardness linearly increases from its central point toward itssurface. In the sphere 16 whose hardness linearly increases, the energyloss is low when the golf ball 2 is hit with a middle iron.

The inner cover 8 preferably has a thickness of 0.2 mm or greater but2.0 mm or less. In the sphere 16 that includes the inner cover 8 havinga thickness of 0.2 mm or greater, an outer-hard/inner-soft structure canbe achieved. In this respect, the thickness is more preferably equal toor greater than 0.5 mm and particularly preferably equal to or greaterthan 0.8 mm. The golf ball 2 that includes the inner cover 8 having athickness of 2.0 mm or less has excellent resilience performance. Inthis respect, the thickness is more preferably equal to or less than 1.6mm and particularly preferably equal to or less than 1.3 mm.

In light of feel at impact, the sphere 16 consisting of the core 4 andthe inner cover 8 has an amount of compressive deformation Di ofpreferably 3.2 mm or greater and particularly preferably 3.4 mm orgreater. In light of resilience performance, the amount of compressivedeformation Di is preferably equal to or less than 3.8 mm andparticularly preferably equal to or less than 3.6 mm.

For forming the inner cover 8, known methods such as injection molding,compression molding, and the like can be used.

For the outer cover 10, a resin composition is suitably used. Apreferable base polymer of the resin composition is an ionomer resin.The golf ball 2 that includes the outer cover 10 including the ionomerresin has excellent resilience performance. The ionomer resin describedabove for the inner cover 8 can be used for the outer cover 10.

An ionomer resin and another resin may be used in combination. In thiscase, in light of resilience performance, the ionomer resin is includedas the principal component of the base polymer. The proportion of theionomer resin to the entire base polymer is preferably equal to orgreater than 50% by weight, more preferably equal to or greater than 60%by weight, and particularly preferably equal to or greater than 70% byweight.

A preferable resin that can be used in combination with an ionomer resinis an ethylene-(meth)acrylic acid copolymer. The copolymer is obtainedby a copolymerization reaction of a monomer composition that containsethylene and (meth)acrylic acid. In the copolymer, some of the carboxylgroups are neutralized with metal ions. The copolymer includes 3% byweight or greater but 25% by weight or less of a (meth)acrylic acidcomponent. An ethylene-(meth)acrylic acid copolymer having a polarfunctional group is particularly preferred. A specific example ofethylene-(meth)acrylic acid copolymers is trade name “NUCREL”manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

Another preferable resin that can be used in combination with an ionomerresin is a styrene block-containing thermoplastic elastomer. The styreneblock-containing thermoplastic elastomer described above for the innercover 8 can be used for the outer cover 10.

According to need, a coloring agent such as titanium dioxide and afluorescent pigment, a filler such as barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial, a fluorescent brightener, and the like are included in theouter cover 10 in an adequate amount.

The outer cover 10 preferably has a JIS-C hardness Ho of 83 or greaterbut 96 or less. In the golf ball 2 that includes the outer cover 10having a hardness Ho of 83 or greater, an outer-hard/inner-softstructure can be achieved. In the golf ball 2 that has theouter-hard/inner-soft structure, spin is suppressed. The golf ball 2 hasexcellent flight performance. In this respect, the hardness Ho is morepreferably equal to or greater than 84 and particularly preferably equalto or greater than 85. The golf ball 2 that includes the outer cover 10having a hardness Ho of 96 or less has excellent feel at impact. In thisrespect, the hardness Ho is more preferably equal to or less than 95 andparticularly preferably equal to or less than 93. The hardness Ho ismeasured by the same measurement method as that for the hardness Hi.

The outer cover 10 preferably has a thickness of 0.2 mm or greater but1.5 mm or less. The outer cover 10 having a thickness of 0.2 mm orgreater can easily be formed. In this respect, the thickness is morepreferably equal to or greater than 0.4 mm and particularly preferablyequal to or greater than 0.6 mm. In the golf ball 2 that includes theouter cover 10 having a thickness of 1.5 mm or less, spin is suppressed.In this respect, the thickness is more preferably equal to or less than1.3 mm and particularly preferably equal to or less than 1.1 mm.

For forming the outer cover 10, known methods such as injection molding,compression molding, and the like can be used. When forming the outercover 10, the dimples 12 are formed by pimples formed on the cavity faceof a mold.

The cover 6 preferably has a total thickness of 2.5 mm or less. The golfball 2 that includes the cover 6 having a total thickness of 2.5 mm orless has excellent feel at impact. In this respect, the total thicknessis more preferably equal to or less than 2.3 mm and particularlypreferably equal to or less than 2.1 mm. In light of durability of thegolf ball 2, the total thickness is preferably equal to or greater than0.3 mm, more preferably equal to or greater than 0.5 mm, andparticularly preferably equal to or greater than 0.8 mm.

The JIS-C hardness Ho of the outer cover 10 is greater than the JIS-Chardness Hi of the inner cover 8. The outer cover 10 can achieve anouter-hard/inner-soft structure of the golf ball 2. The golf ball 2 hasexcellent flight performance and excellent feel at impact. Thedifference (Ho−Hi) is preferably equal to or greater than 2, morepreferably equal to or greater than 4, and particularly preferably equalto or greater than 6. In light of suppression of energy loss when thegolf ball 2 is hit, the difference (Ho−Hi) is preferably equal to orless than 10.

In a hardness distribution curve of the golf ball 2 from the centralpoint of the core 4 to the outer cover 10, the hardness of the outercover 10 is the greatest. In the golf ball 2, spin is suppressed.

In light of feel at impact, the golf ball 2 has an amount of compressivedeformation Db of preferably 2.8 mm or greater, more preferably 2.9 mmor greater, and particularly preferably 3.0 mm or greater. In light ofresilience performance, the amount of compressive deformation Db ispreferably equal to or less than 3.6 mm, more preferably equal to orless than 3.5 mm, and particularly preferably equal to or less than 3.4mm.

At measurement of the amount of compressive deformation, first, a spheresuch as the core 4, the golf ball 2, or the like is placed on a hardplate made of metal. Next, a cylinder made of metal gradually descendstoward the sphere. The sphere, squeezed between the bottom face of thecylinder and the hard plate, becomes deformed. A migration distance ofthe cylinder, starting from the state in which an initial load of 98 Nis applied to the sphere up to the state in which a final load of 1274 Nis applied thereto, is measured.

Second Embodiment

A golf ball 102 shown in FIG. 3 includes a spherical core 104 and acover 106 covering the core 104. The cover 106 includes an inner cover108 and an outer cover 110 positioned outside the inner cover 108. Theinner cover 108 is an innermost layer of the cover 106. The outer cover110 is an outermost layer of the cover 106. The cover 106 may includeanother one or more layers between the inner cover 108 and the outercover 110. On the surface of the outer cover 110, a large number ofdimples 112 are formed. Of the surface of the golf ball 102, a partother than the dimples 112 is a land 114. The golf ball 102 includes apaint layer and a mark layer on the external side of the outer cover110, but these layers are not shown in the drawing.

The golf ball 102 has a diameter of 40 mm or greater but 45 mm or less.From the standpoint of conformity to the rules established by the UnitedStates Golf Association (USGA), the diameter is preferably equal to orgreater than 42.67 mm. In light of suppression of air resistance, thediameter is preferably equal to or less than 44 mm and more preferablyequal to or less than 42.80 mm. The golf ball 102 has a weight of 40 gor greater but 50 g or less. In light of attainment of great inertia,the weight is preferably equal to or greater than 44 g and morepreferably equal to or greater than 45.00 g. From the standpoint ofconformity to the rules established by the USGA, the weight ispreferably equal to or less than 45.93 g.

FIG. 4 is a line graph showing a hardness distribution of the core 104of the golf ball 102 in FIG. 3. The horizontal axis of the graphindicates the ratio (%) of a distance from the central point of the core104 to the radius of the core 104. The vertical axis of the graphindicates a JIS-C hardness. Nine measuring points obtained by dividing aregion from the central point of the core 104 to the surface of the core104 at intervals of 12.5% of the radius of the core 104 are plotted inthe graph. The ratio of the distance from the central point of the core104 to each of these measuring points to the radius of the core 104 isas follows.

First point: 0.0% (central point)

Second point: 12.5%

Third point: 25.0%

Fourth point: 37.5%

Fifth point: 50.0%

Sixth point: 62.5%

Seventh point: 75.0%

Eighth point: 87.5%

Ninth point: 100.0% (surface)

Hardnesses at the first to eighth points are measured by pressing aJIS-C type hardness scale against a cut plane of the core 104 that hasbeen cut into two halves. A hardness Hs at the ninth point is measuredby pressing the JIS-C type hardness scale against the surface of thespherical core 104. For the measurement, an automated rubber hardnessmeasurement machine (trade name “P1”, manufactured by Kobunshi KeikiCo., Ltd.), to which this hardness scale is mounted, is used. In thepresent invention, a JIS-C hardness at a measuring point whose distancefrom the central point of the core 104 is x (%) is represented by H(x).The hardness at the central point of the core 104 is represented byH(0).

FIG. 4 also shows a linear approximation curve obtained by aleast-square method on the basis of the distances and the hardnesses ofthe nine measuring points. As is clear from FIG. 4, the broken line doesnot greatly deviate from the linear approximation curve. In other words,the broken line has a shape close to the linear approximation curve. Inthe core 104, the hardness linearly increases from its central pointtoward its surface. When the core 104 is hit with a driver, the energyloss is low. The core 104 has excellent resilience performance. In thegolf ball 102 that includes the core 104, spin is suppressed. When thegolf ball 102 is hit with a driver, the flight distance is large.

In the core 104, R² of the linear approximation curve obtained by theleast-square method is equal to or greater than 0.95. R² is an indexindicating the linearity of the broken line. For the core 104 for whichR² is equal to or greater than 0.95, the shape of the broken line of thehardness distribution is close to a straight line. The core 104 forwhich R² is equal to or greater than 0.95 has excellent resilienceperformance. R² is more preferably equal to or greater than 0.96 andparticularly preferably equal to or greater than 0.97. R² is calculatedby squaring a correlation coefficient R. The correlation coefficient Ris calculated by dividing the covariance of the distance (%) from thecentral point and the hardness (JIS-C) by the standard deviation of thedistance (%) from the central point and the standard deviation of thehardness (JIS-C).

The core 104 is obtained by crosslinking a rubber composition. Therubber composition includes:

(a) a base rubber;

(b) a co-crosslinking agent;

(c) a crosslinking initiator; and

(d) an acid and/or a salt.

During heating and forming of the core 104, the base rubber (a) iscrosslinked by the co-crosslinking agent (b). The heat of thecrosslinking reaction remains near the central point of the core 104.Thus, during heating and forming of the core 104, the temperature at thecentral portion is high. The temperature gradually decreases from thecentral point toward the surface. It is inferred that in the rubbercomposition, the acid reacts with a metal salt of the co-crosslinkingagent (b) to bond to cation. It is inferred that in the rubbercomposition, the salt reacts with the metal salt of the co-crosslinkingagent (b) to exchange cation. By the bonding and the exchange, metallicbonding is broken. This breaking is likely to occur in the centralportion of the core 104 where the temperature is high, and is unlikelyto occur near the surface of the core 104. As a result, the crosslinkingdensity of the core 104 increases from its central point toward itssurface. In the core 104, an outer-hard/inner-soft structure can beachieved. Furthermore, when the rubber composition includes an organicsulfur compound (e) together with the acid and/or the salt (d), thegradient of the hardness distribution can be controlled, and the degreeof the outer-hard/inner-soft structure of the core 104 can be increased.When the golf ball 102 that includes the core 104 is hit with a driver,the spin rate is low. In the golf ball 102, excellent flight performanceis achieved upon a shot with a driver.

The rubber composition of the core 104 can include, as the base rubber(a), the base rubber (a) described above for the rubber composition ofthe core 4 according to the first embodiment.

The rubber composition of the core 104 can include, as theco-crosslinking agent (b), the co-crosslinking agent (b) described abovefor the rubber composition of the core 4 according to the firstembodiment. The co-crosslinking agent (b) is:

(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or

(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.

The rubber composition may include only the α,β-unsaturated carboxylicacid (b1) or only the metal salt (b2) of the α,β-unsaturated carboxylicacid as the co-crosslinking agent (b). The rubber composition mayinclude both the α,β-unsaturated carboxylic acid (b1) and the metal salt(b2) of the α,β-unsaturated carboxylic acid as the co-crosslinking agent(b).

The metal salt (b2) of the α,β-unsaturated carboxylic acidgraft-polymerizes with the molecular chain of the base rubber, therebycrosslinking the rubber molecules. When the rubber composition includesthe α,β-unsaturated carboxylic acid (b1), the rubber compositionpreferably further includes a metal compound (f). The metal compound (f)reacts with the α,β-unsaturated carboxylic acid (b1) in the rubbercomposition. A salt obtained by this reaction graft-polymerizes with themolecular chain of the base rubber.

The rubber composition of the core 104 can include, as the metalcompound (f), the metal compound (f) described above for the rubbercomposition of the core 4 according to the first embodiment.

In light of resilience performance of the golf ball 102, the amount ofthe co-crosslinking agent (b) is preferably equal to or greater than 15parts by weight and particularly preferably equal to or greater than 20parts by weight, per 100 parts by weight of the base rubber. In light offeel at impact, the amount is preferably equal to or less than 50 partsby weight, more preferably equal to or less than 45 parts by weight, andparticularly preferably equal to or less than 40 parts by weight, per100 parts by weight of the base rubber.

The rubber composition of the core 104 can include, as the crosslinkinginitiator (c), the crosslinking initiator (c) described above for therubber composition of the core 4 according to the first embodiment.

In light of resilience performance of the golf ball 102, the amount ofthe crosslinking initiator (c) is preferably equal to or greater than0.2 parts by weight and particularly preferably equal to or greater than0.5 parts by weight, per 100 parts by weight of the base rubber. Inlight of feel at impact and durability of the golf ball 102, the amountis preferably equal to or less than 5.0 parts by weight and particularlypreferably equal to or less than 2.5 parts by weight, per 100 parts byweight of the base rubber.

The rubber composition of the core 104 can include, as the acid and/orthe salt (d), the acid and/or the salt (d) described above for therubber composition of the core 4 according to the first embodiment. Inthe present invention, the co-crosslinking agent (b) is not included inthe concept of the acid and/or the salt (d). It is inferred that asdescribed above, the acid and/or the salt (d) breaks the metalcrosslinks by the co-crosslinking agent (b) in the central portion ofthe core 104 during heating and forming of the core 104. Examples of theacid and/or the salt (d) include oxo acids, such as carboxylic acids,sulfonic acids, and phosphoric acid, and salts thereof; and hydroacids,such as hydrochloric acid and hydrofluoric acid, and salts thereof. Oxoacids and salts thereof are preferred. A carboxylic acid and/or a saltthereof (d1) is more preferred. Carboxylates are particularly preferred.

The carboxylic acid component of the carboxylic acid and/or the saltthereof (d1) has a carboxyl group. The carbon number of the carboxylicacid component of the carboxylic acid and/or the salt thereof (d1) ispreferably equal to or greater than 1 but equal to or less than 30, morepreferably equal to or greater than 3 but equal to or less than 30, andeven more preferably equal to or greater than 5 but equal to or lessthan 28. Examples of the carboxylic acid include aliphatic carboxylicacids (fatty acids) and aromatic carboxylic acids. Fatty acids and saltsthereof are preferred.

The rubber composition may include a saturated fatty acid or a saltthereof, or may include an unsaturated fatty acid or a salt thereof. Thesaturated fatty acid and the salt thereof are preferred.

Examples of preferable carboxylates include a potassium salt, amagnesium salt, an aluminum salt, a zinc salt, an iron salt, a coppersalt, a nickel salt, or a cobalt salt of octanoic acid, lauric acid,myristic acid, palmitic acid, stearic acid, oleic acid, or behenic acid.Zinc salts of carboxylic acids are particularly preferred. Specificexamples of preferable carboxylates include zinc octoate, zinc laurate,zinc myristate, and zinc stearate.

In light of linearity of the hardness distribution of the core 104, theamount of the acid and/or the salt (d) is preferably equal to or greaterthan 0.5 parts by weight, more preferably equal to or greater than 1.0parts by weight, even more preferably equal to or greater than 1.5 partsby weight, and particularly preferably equal to or greater than 2.0parts by weight, per 100 parts by weight of the base rubber. In light ofresilience performance, the amount is preferably equal to or less than40 parts by weight, more preferably less than 40 parts by weight, evenmore preferably equal to or less than 30 parts by weight, andparticularly preferably equal to or less than 20 parts by weight, per100 parts by weight of the base rubber.

The weight ratio of the co-crosslinking agent (b) and the acid and/orthe salt (d) in the rubber composition is preferably equal to or greaterthan 3/7 but equal to or less than 9/1, and is particularly preferablyequal to or greater than 4/6 but equal to or less than 8/2. From therubber composition in which this weight ratio is within the above range,the core 104 whose hardness linearly increases from its central pointtoward its surface can be obtained.

As the co-crosslinking agent (b), zinc acrylate is preferably used. Zincacrylate whose surface is coated with stearic acid or zinc stearate forthe purpose of improving dispersibility to rubber is present. In thepresent invention, when the rubber composition includes this zincacrylate, this coating material is not included in the concept of theacid and/or the salt (d).

The rubber composition preferably further includes an organic sulfurcompound (e). The organic sulfur compound (e) can contribute to controlof: the linearity of the hardness distribution of the core 104; and thedegree of the outer-hard/inner-soft structure. An example of the organicsulfur compound (e) is an organic compound having a thiol group or apolysulfide linkage having 2 to 4 sulfur atoms. A metal salt of thisorganic compound is also included in the organic sulfur compound (e).The rubber composition of the core 104 can include, as the organicsulfur compound (e), the organic sulfur compound (e) described above forthe rubber composition of the core 4 according to the first embodiment.

In light of resilience performance, the amount of the organic sulfurcompound (e) is preferably equal to or greater than 0.05 parts by weightand particularly preferably equal to or greater than 0.1 parts byweight, per 100 parts by weight of the base rubber. In light ofresilience performance, the amount is preferably equal to or less than5.0 parts by weight and particularly preferably equal to or less than2.0 parts by weight, per 100 parts by weight of the base rubber.

For the purpose of adjusting specific gravity and the like, a filler maybe included in the core 104. Examples of suitable fillers include zincoxide, barium sulfate, calcium carbonate, and magnesium carbonate. Theamount of the filler is determined as appropriate so that the intendedspecific gravity of the core 104 is accomplished. A particularlypreferable filler is zinc oxide. Zinc oxide serves not only as aspecific gravity adjuster but also as a crosslinking activator.

According to need, an anti-aging agent, a coloring agent, a plasticizer,a dispersant, sulfur, a vulcanization accelerator, and the like areadded to the rubber composition of the core 104. Crosslinked rubberpowder or synthetic resin powder may also be dispersed in the rubbercomposition.

In the core 104, the difference (Hs−H(0)) between the surface hardnessHs and the central hardness H(0) is preferably equal to or greater than15. The difference is great. In other words, the core 104 has anouter-hard/inner-soft structure. When the core 104 is hit with a driver,the recoil (torsional return) is great, and thus spin is suppressed. Thecore 104 contributes to the flight performance of the golf ball 102. Inlight of flight performance, the difference (Hs−H(0)) is more preferablyequal to or greater than 20 and particularly preferably equal to orgreater than 25. From the standpoint that the core 104 can easily beformed, the difference (Hs−H(0)) is preferably equal to or less than 50.

The hardness H(0) at the central point of the core 104 is preferablyequal to or greater than 40.0 but equal to or less than 70.0. The golfball 102 having a hardness H(0) of 40.0 or greater has excellentresilience performance. In this respect, the hardness H(0) is morepreferably equal to or greater than 45.0 and particularly preferablyequal to or greater than 50.0. The core 104 having a hardness H(0) of70.0 or less can achieve an outer-hard/inner-soft structure. In the golfball 102 that includes the core 104, spin can be suppressed. In thisrespect, the hardness H(0) is more preferably equal to or less than 68.0and particularly preferably equal to or less than 66.0.

The hardness Hs at the surface of the core 104 is preferably equal to orgreater than 78.0 but equal to or less than 95.0. In the core 104 havinga hardness Hs of 78.0 or greater, an outer-hard/inner-soft structure canbe achieved. In the golf ball 102 that includes the core 104, spin canbe suppressed.

In this respect, the hardness Hs is more preferably equal to or greaterthan 80.0 and particularly preferably equal to or greater than 82.0. Thegolf ball 102 having a hardness Hs of 95.0 or less has excellentdurability. In this respect, the hardness Hs is more preferably equal toor less than 93.0 and particularly preferably equal to or less than90.0.

The core 104 preferably has a diameter of 38.0 mm or greater but 42.0 mmor less. The core 104 having a diameter of 38.0 mm or greater canachieve excellent resilience performance of the golf ball 102. In thisrespect, the diameter is more preferably equal to or greater than 38.5mm and particularly preferably equal to or greater than 39.0 mm. In thegolf ball 102 that includes the core 104 having a diameter of 42.0 mm orless, the inner cover 108 and the outer cover 110 can have sufficientthicknesses. The golf ball 102 that includes the inner cover 108 and theouter cover 110 which have large thicknesses has excellent durability.In this respect, the diameter is more preferably equal to or less than41.0 mm and particularly preferably equal to or less than 40.0 mm.

In light of feel at impact, the core 104 has an amount of compressivedeformation Dc of preferably 3.0 mm or greater and particularlypreferably 3.3 mm or greater. In light of resilience performance, theamount of compressive deformation Dc is preferably equal to or less than4.6 mm and particularly preferably equal to or less than 4.3 mm.

For the inner cover 108, a resin composition is suitably used. Examplesof the base polymer of the resin composition include ionomer resins,styrene block-containing thermoplastic elastomers, thermoplasticpolyester elastomers, thermoplastic polyamide elastomers, andthermoplastic polyolefin elastomers.

Particularly preferable base polymers are ionomer resins. The golf ball102 that includes the inner cover 108 including an ionomer resin hasexcellent resilience performance. An ionomer resin and another resin maybe used in combination for the inner cover 108. In this case, theprincipal component of the base polymer is preferably the ionomer resin.Specifically, the proportion of the ionomer resin to the entire basepolymer is preferably equal to or greater than 50% by weight, morepreferably equal to or greater than 60% by weight, and particularlypreferably equal to or greater than 70% by weight.

Examples of preferable ionomer resins include binary copolymers formedwith an α-olefin and an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms. A preferable binary copolymer includes 80% by weight orgreater but 90% by weight or less of an α-olefin, and 10% by weight orgreater but 20% by weight or less of an α,β-unsaturated carboxylic acid.The binary copolymer has excellent resilience performance. Examples ofother preferable ionomer resins include ternary copolymers formed with:an α-olefin; an α,β-unsaturated carboxylic acid having 3 to 8 carbonatoms; and an α,β-unsaturated carboxylate ester having 2 to 22 carbonatoms. A preferable ternary copolymer includes 70% by weight or greaterbut 85% by weight or less of an α-olefin, 5% by weight or greater but30% by weight or less of an α,β-unsaturated carboxylic acid, and 1% byweight or greater but 25% by weight or less of an α,β-unsaturatedcarboxylate ester. The ternary copolymer has excellent resilienceperformance. For the binary copolymers and the ternary copolymers,preferable α-olefins are ethylene and propylene, while preferableα,β-unsaturated carboxylic acids are acrylic acid and methacrylic acid.A particularly preferable ionomer resin is a copolymer formed withethylene and acrylic acid or methacrylic acid.

In the binary copolymers and the ternary copolymers, some of thecarboxyl groups are neutralized with metal ions. Examples of metal ionsfor use in neutralization include sodium ion, potassium ion, lithiumion, zinc ion, calcium ion, magnesium ion, aluminum ion, and neodymiumion. The neutralization may be carried out with two or more types ofmetal ions. Particularly suitable metal ions in light of resilienceperformance and durability of the golf ball 2 are sodium ion, zinc ion,lithium ion, and magnesium ion.

Specific examples of ionomer resins include trade names “Himilan 1555”,“Himilan 1557”, “Himilan 1605”, “Himilan 1706”, “Himilan 1707”, “Himilan1856”, “Himilan 1855”, “Himilan AM7311”, “HimilanAM7315”, “HimilanAM7317”, “HimilanAM7318”, “Himilan AM7329”, “Himilan AM7337”, “HimilanMK7320”, and “Himilan MK7329”, manufactured by Du Pont-MITSUIPOLYCHEMICALS Co., Ltd.; trade names “Surlyn 6120”, “Surlyn 6910”,“Surlyn 7930”, “Surlyn 7940”, “Surlyn 8140”, “Surlyn 8150”, “Surlyn8940”, “Surlyn 8945”, “Surlyn 9120”, “Surlyn 9150”, “Surlyn 9910”,“Surlyn 9945”, “Surlyn AD8546”, “HPF1000”, and “HPF2000”, manufacturedby E. I. du Pont de Nemours and Company; and trade names “IOTEK 7010”,“IOTEK 7030”, “IOTEK 7510”, “IOTEK 7520”, “IOTEK 8000”, and “IOTEK8030”, manufactured by ExxonMobil Chemical Company.

Two or more ionomer resins may be used in combination for the innercover 108. An ionomer resin neutralized with a monovalent metal ion, andan ionomer resin neutralized with a bivalent metal ion may be used incombination.

A preferable resin that can be used in combination with an ionomer resinis a styrene block-containing thermoplastic elastomer. The styreneblock-containing thermoplastic elastomer has excellent compatibilitywith ionomer resins. A resin composition including the styreneblock-containing thermoplastic elastomer has excellent fluidity.

The styrene block-containing thermoplastic elastomer includes apolystyrene block as a hard segment, and a soft segment. A typical softsegment is a diene block. Examples of compounds for the diene blockinclude butadiene, isoprene, 1,3-pentadiene, and2,3-dimethyl-1,3-butadiene. Butadiene and isoprene are preferred. Two ormore compounds may be used in combination.

Examples of styrene block-containing thermoplastic elastomers includestyrene-butadiene-styrene block copolymers (SBS),styrene-isoprene-styrene block copolymers (SIS),styrene-isoprene-butadiene-styrene block copolymers (SIBS), hydrogenatedSBS, hydrogenated SIS, and hydrogenated SIBS. Examples of hydrogenatedSBS include styrene-ethylene-butylene-styrene block copolymers (SEBS).Examples of hydrogenated SIS include styrene-ethylene-propylene-styreneblock copolymers (SEPS). Examples of hydrogenated SIBS includestyrene-ethylene-ethylene-propylene-styrene block copolymers (SEEPS).

In light of resilience performance of the golf ball 102, the content ofthe styrene component in the styrene block-containing thermoplasticelastomer is preferably equal to or greater than 10% by weight, morepreferably equal to or greater than 12% by weight, and particularlypreferably equal to or greater than 15% by weight. In light of feel atimpact of the golf ball 102, the content is preferably equal to or lessthan 50% by weight, more preferably equal to or less than 47% by weight,and particularly preferably equal to or less than 45% by weight.

In the present invention, styrene block-containing thermoplasticelastomers include an alloy of an olefin and one or more membersselected from the group consisting of SBS, SIS, SIBS, SEBS, SEPS, SEEPS,and hydrogenated products thereof. The olefin component in the alloy ispresumed to contribute to improvement of compatibility with ionomerresins. Use of this alloy improves the resilience performance of thegolf ball 102. An olefin having 2 to 10 carbon atoms is preferably used.Examples of suitable olefins include ethylene, propylene, butene, andpentene. Ethylene and propylene are particularly preferred.

Specific examples of polymer alloys include trade names “RabalonT3221C”,“RabalonT3339C”, “RabalonSJ4400N”, “Rabalon SJ5400N”, “Rabalon SJ6400N”,“Rabalon SJ7400N”, “Rabalon SJ8400N”, “Rabalon SJ9400N”, and “RabalonSR04”, manufactured by Mitsubishi Chemical Corporation. Other specificexamples of styrene block-containing thermoplastic elastomers includetrade name “Epofriend A1010” manufactured by Daicel Chemical Industries,Ltd., and trade name “Septon HG-252” manufactured by Kuraray Co., Ltd.

According to need, a coloring agent such as titanium dioxide and afluorescent pigment, a filler such as barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial, a fluorescent brightener, and the like are included in theresin composition of the inner cover 108 in an adequate amount. Theinner cover 108 may include powder of a metal with a high specificgravity.

In the golf ball 102, the inner cover 108 has a hardness Hi which isequal to or less than the JIS-C hardness Hs at the surface of the core104. When the golf ball 102 is hit with a driver, the inner cover 108achieves soft feel at impact. In light of feel at impact, the difference(Hi−Hs) is preferably equal to or greater than 1, more preferably equalto or greater than 2, and particularly preferably equal to or greaterthan 7. When the difference (Hi−Hs) is not excessively great, light feelat impact is obtained. In this respect, the difference (Hi−Hs) ispreferably equal to or less than 20, more preferably equal to or lessthan 18, and particularly preferably equal to or less than 12.

In light of soft feel at impact, the hardness Hi of the inner cover 108is preferably equal to or less than 90, more preferably equal to or lessthan 85, and particularly preferably equal to or less than 83. In lightof light feel at impact, the hardness Hi is preferably equal to orgreater than 60, more preferably equal to or greater than 65, andparticularly preferably equal to or greater than 71.

The hardness Hi is measured with a JIS-C type hardness scale mounted toan automated rubber hardness measurement machine (trade name “P1”,manufactured by Kobunshi Keiki Co., Ltd.). For the measurement, a slabthat is formed by hot press and that has a thickness of about 2 mm isused. A slab kept at 23° C. for two weeks is used for the measurement.At the measurement, three slabs are stacked. A slab formed from the sameresin composition as the resin composition of the inner cover 108 isused.

The inner cover 108 preferably has a thickness of 0.2 mm or greater but2.0 mm or less. The golf ball 102 that includes the inner cover 108having a thickness of 0.2 mm or greater has excellent feel at impact. Inthis respect, the thickness of the inner cover 108 is more preferablyequal to or greater than 0.5 mm and particularly preferably equal to orgreater than 0.8 mm. The golf ball 102 that includes the inner cover 108having a thickness of 2.0 mm or less has excellent resilienceperformance. In this respect, the thickness is more preferably equal toor less than 1.5 mm and particularly preferably equal to or less than1.2 mm.

In light of feel at impact, a sphere 116 consisting of the core 104 andthe inner cover 108 has an amount of compressive deformation Di ofpreferably 3.2 mm or greater and particularly preferably 3.4 mm orgreater. In light of resilience performance, the amount of compressivedeformation Di is preferably equal to or less than 3.8 mm andparticularly preferably equal to or less than 3.6 mm.

For forming the inner cover 108, known methods such as injectionmolding, compression molding, and the like can be used.

For the outer cover 110, a resin composition is suitably used. Apreferable base polymer of the resin composition is an ionomer resin.The golf ball 102 that includes the outer cover 110 including theionomer resin has excellent resilience performance. The ionomer resindescribed above for the inner cover 108 can be used for the outer cover110.

An ionomer resin and another resin may be used in combination. In thiscase, in light of resilience performance, the ionomer resin is includedas the principal component of the base polymer. The proportion of theionomer resin to the entire base polymer is preferably equal to orgreater than 50% by weight, more preferably equal to or greater than 60%by weight, and particularly preferably equal to or greater than 70% byweight.

A preferable resin that can be used in combination with an ionomer resinis an ethylene-(meth)acrylic acid copolymer. The copolymer is obtainedby a copolymerization reaction of a monomer composition that containsethylene and (meth)acrylic acid. In the copolymer, some of the carboxylgroups are neutralized with metal ions. The copolymer includes 3% byweight or greater but 25% by weight or less of a (meth)acrylic acidcomponent. An ethylene-(meth)acrylic acid copolymer having a polarfunctional group is particularly preferred. A specific example ofethylene-(meth)acrylic acid copolymers is trade name “NUCREL”manufactured by Du Pont-MITSUI POLYCHEMICALS Co., Ltd.

According to need, a coloring agent such as titanium dioxide and afluorescent pigment, a filler such as barium sulfate, a dispersant, anantioxidant, an ultraviolet absorber, a light stabilizer, a fluorescentmaterial, a fluorescent brightener, and the like are included in theouter cover 110 in an adequate amount.

The outer cover 110 preferably has a JIS-C hardness Ho of 83 or greaterbut 96 or less. In the golf ball 102 that includes the outer cover 110having a hardness Ho of 83 or greater, an outer-hard/inner-softstructure can be achieved. In the golf ball 102 that has theouter-hard/inner-soft structure, spin is suppressed. The golf ball 102has excellent flight performance. In this respect, the hardness Ho ismore preferably equal to or greater than 84 and particularly preferablyequal to or greater than 85. The golf ball 102 that includes the outercover 110 having a hardness Ho of 96 or less has excellent feel atimpact. In this respect, the hardness Ho is more preferably equal to orless than 95 and particularly preferably equal to or less than 93. Thehardness Ho is measured by the same measurement method as that for thehardness Hi.

The outer cover 110 preferably has a thickness of 0.2 mm or greater but1.5 mm or less. The outer cover 110 having a thickness of 0.2 mm orgreater can easily be formed. In this respect, the thickness is morepreferably equal to or greater than 0.4 mm and particularly preferablyequal to or greater than 0.6 mm. The golf ball 102 that includes theouter cover 110 having a thickness of 1.5 mm or less has excellent feelat impact. In this respect, the thickness is more preferably equal to orless than 1.3 mm and particularly preferably equal to or less than 1.1mm.

For forming the outer cover 110, known methods such as injectionmolding, compression molding, and the like can be used. When forming theouter cover 110, the dimples 112 are formed by pimples formed on thecavity face of a mold.

The cover 106 preferably has a total thickness of 2.5 mm or less. Thegolf ball 102 that includes the cover 106 having a total thickness of2.5 mm or less has excellent feel at impact. In this respect, the totalthickness is more preferably equal to or less than 2.3 mm andparticularly preferably equal to or less than 2.1 mm. In light ofdurability of the golf ball 102, the total thickness is preferably equalto or greater than 0.3 mm, more preferably equal to or greater than 0.5mm, and particularly preferably equal to or greater than 0.8 mm.

The JIS-C hardness Ho of the outer cover 110 is greater than the JIS-Chardness Hi of the inner cover 108. The outer cover 110 can achieve anouter-hard/inner-soft structure of the golf ball 102. The golf ball 102has excellent flight performance and excellent feel at impact. Thedifference (Ho−Hi) is preferably equal to or greater than 5, morepreferably equal to or greater than 9, and particularly preferably equalto or greater than 16. In light of suppression of energy loss when thegolf ball 102 is hit, the difference (Ho−Hi) is preferably equal to orless than 27 and particularly preferably equal to or less than 21.

In a hardness distribution curve of the golf ball 102 from the centralpoint of the core 104 to the outer cover 110, the hardness of the outercover 110 is the greatest. In the golf ball 102, spin is suppressed.

In light of feel at impact, the golf ball 102 has an amount ofcompressive deformation Db of preferably 2.8 mm or greater, morepreferably 2.9 mm or greater, and particularly preferably 3.0 mm orgreater. In light of resilience performance, the amount of compressivedeformation Db is preferably equal to or less than 3.6 mm, morepreferably equal to or less than 3.5 mm, and particularly preferablyequal to or less than 3.4 mm.

At measurement of the amount of compressive deformation, first, a spheresuch as the core 104, the golf ball 102, or the like is placed on a hardplate made of metal. Next, a cylinder made of metal gradually descendstoward the sphere. The sphere, squeezed between the bottom face of thecylinder and the hard plate, becomes deformed. A migration distance ofthe cylinder, starting from the state in which an initial load of 98 Nis applied to the sphere up to the state in which a final load of 1274 Nis applied thereto, is measured.

Preferred embodiments of the invention are specified in the followingparagraphs:

1. A golf ball comprising a spherical core and a cover covering the coreand including two or more layers, wherein

when distances (%) from a central point of the core to nine points andJIS-C hardnesses at the nine points, which nine points are obtained bydividing a region from the central point of the core to a surface of thecore at intervals of 12.5% of a radius of the core, are plotted in agraph, R² of a linear approximation curve obtained by a least-squaremethod is equal to or greater than 0.95, and

a JIS-C hardness Hi of an innermost layer of the cover is greater than aJIS-C hardness Hs at the surface of the core.

2. The golf ball according to paragraph 1, wherein a difference(Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0) at thecentral point of the core is equal to or greater than 15.3. The golf ball according to paragraph 1, wherein a difference (Hi−Hs)between the hardness Hi and the hardness Hs is equal to or greater than1 but equal to or less than 5.4. The golf ball according to paragraph 1, wherein a difference (Ho−Hi)between a JIS-C hardness Ho of an outermost layer of the cover and theJIS-C hardness Hi of the innermost layer of the cover is equal to orgreater than 2 but equal to or less than 10.5. The golf ball according to paragraph 1, wherein, in a hardnessdistribution curve from the central point of the core to an outermostlayer of the cover, a hardness of the outermost layer is the greatest.6. The golf ball according to paragraph 1, wherein a total thickness ofthe cover is equal to or less than 2.5 mm.7. The golf ball according to paragraph 1, wherein

the core is formed by a rubber composition being crosslinked,

the rubber composition includes:

-   -   (a) a base rubber;    -   (b) a co-crosslinking agent;    -   (c) a crosslinking initiator; and    -   (d) an acid and/or a salt, and

the co-crosslinking agent (b) is:

-   -   (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon        atoms; or    -   (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3        to 8 carbon atoms.        8. The golf ball according to paragraph 7, wherein an amount of        the acid and/or the salt (d) is equal to or greater than 0.5        parts by weight but equal to or less than 40 parts by weight,        per 100 parts by weight of the base rubber (a).        9. The golf ball according to paragraph 7, wherein the acid        and/or the salt (d) is a carboxylic acid and/or a salt thereof        (d1).        10. The golf ball according to paragraph 9, wherein a carbon        number of a carboxylic acid component of the carboxylic acid        and/or the salt thereof (d1) is equal to or greater than 1 but        equal to or less than 30.        11. The golf ball according to paragraph 9, wherein the        carboxylic acid and/or the salt thereof (d1) is a fatty acid        and/or a salt thereof.        12. The golf ball according to paragraph 9, wherein the        carboxylic acid and/or the salt thereof (d1) is a zinc salt of a        carboxylic acid.        13. The golf ball according to paragraph 12, wherein the zinc        salt of the carboxylic acid is one or more members selected from        the group consisting of zinc octoate, zinc laurate, zinc        myristate, and zinc stearate.        14. The golf ball according to paragraph 7, wherein the rubber        composition further includes an organic sulfur compound (e).        15. The golf ball according to paragraph 7, wherein

the rubber composition includes the α,β-unsaturated carboxylic acid(b1), and

the rubber composition further includes a metal compound (f).

16. The golf ball according to paragraph 7, wherein the rubbercomposition includes the metal salt (b2) of the α,β-unsaturatedcarboxylic acid.17. The golf ball according to paragraph 14, wherein the organic sulfurcompound (e) is at least one member selected from the group consistingof thiophenols, polysulfides having 2 to 4 sulfur atoms, thionaphthols,thiurams, and metal salts thereof.18. The golf ball according to paragraph 7, wherein the rubbercomposition includes 15 parts by weight or greater but 50 parts byweight or less of the co-crosslinking agent (b) per 100 parts by weightof the base rubber (a).19. The golf ball according to paragraph 7, wherein the rubbercomposition includes 0.2 parts by weight or greater but 5.0 parts byweight or less of the crosslinking initiator (c) per 100 parts by weightof the base rubber (a).20. The golf ball according to paragraph 14, wherein the rubbercomposition includes 0.05 parts by weight or greater but 5 parts byweight or less of the organic sulfur compound (e) per 100 parts byweight of the base rubber (a).21. A golf ball comprising a spherical core and a cover covering thecore and including two or more layers, wherein

when distances (%) from a central point of the core to nine points andJIS-C hardnesses at the nine points, which nine points are obtained bydividing a region from the central point of the core to a surface of thecore at intervals of 12.5% of a radius of the core, are plotted in agraph, R² of a linear approximation curve obtained by a least-squaremethod is equal to or greater than 0.95, and

a JIS-C hardness Hi of an innermost layer of the cover is equal to orless than a JIS-C hardness Hs at the surface of the core.

22. The golf ball according to paragraph 21, wherein a difference(Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0) at thecentral point of the core is equal to or greater than 15.23. The golf ball according to paragraph 21, wherein a difference(Hs−Hi) between the hardness Hs and the hardness Hi is equal to orgreater than 1 but equal to or less than 20.24. The golf ball according to paragraph 21, wherein a difference(Ho−Hi) between a JIS-C hardness Ho of an outermost layer of the coverand the JIS-C hardness Hi of the innermost layer of the cover is equalto or greater than 5 but equal to or less than 30.25. The golf ball according to paragraph 21, wherein, in a hardnessdistribution curve from the central point of the core to an outermostlayer of the cover, a hardness of the outermost layer is the greatest.26. The golf ball according to paragraph 21, wherein a total thicknessof the cover is equal to or less than 2.5 mm.27. The golf ball according to paragraph 21, wherein

the core is formed by a rubber composition being crosslinked,

the rubber composition includes:

-   -   (a) a base rubber;    -   (b) a co-crosslinking agent;    -   (c) a crosslinking initiator; and    -   (d) an acid and/or a salt, and

the co-crosslinking agent (b) is:

-   -   (b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon        atoms; or    -   (b2) a metal salt of an α,β-unsaturated carboxylic acid having 3        to 8 carbon atoms.        28. The golf ball according to paragraph 27, wherein an amount        of the acid and/or the salt (d) is equal to or greater than 0.5        parts by weight but equal to or less than 40 parts by weight,        per 100 parts by weight of the base rubber (a).        29. The golf ball according to paragraph 27, wherein the acid        and/or the salt (d) is a carboxylic acid and/or a salt thereof        (d1).        30. The golf ball according to paragraph 29, wherein a carbon        number of a carboxylic acid component of the carboxylic acid        and/or the salt thereof (d1) is equal to or greater than 1 but        equal to or less than 30.        31. The golf ball according to paragraph 29, wherein the        carboxylic acid and/or the salt thereof (d1) is a fatty acid        and/or a salt thereof.        32. The golf ball according to paragraph 29, wherein the        carboxylic acid and/or the salt thereof (d1) is a zinc salt of a        carboxylic acid.        33. The golf ball according to paragraph 32, wherein the zinc        salt of the carboxylic acid is one or more members selected from        the group consisting of zinc octoate, zinc laurate, zinc        myristate, and zinc stearate.        34. The golf ball according to paragraph 27, wherein the rubber        composition further includes an organic sulfur compound (e).        35. The golf ball according to paragraph 27, wherein

the rubber composition includes the α,β-unsaturated carboxylic acid(b1), and

the rubber composition further includes a metal compound f.

36. The golf ball according to paragraph 27, wherein the rubbercomposition includes the metal salt (b2) of the α,β-unsaturatedcarboxylic acid.37. The golf ball according to paragraph 34, wherein the organic sulfurcompound (e) is at least one member selected from the group consistingof thiophenols, polysulfides having 2 to 4 sulfur atoms, thionaphthols,thiurams, and metal salts thereof.38. The golf ball according to paragraph 27, wherein the rubbercomposition includes 15 parts by weight or greater but 50 parts byweight or less of the co-crosslinking agent (b) per 100 parts by weightof the base rubber (a).39. The golf ball according to paragraph 27, wherein the rubbercomposition includes 0.2 parts by weight or greater but 5.0 parts byweight or less of the crosslinking initiator (c) per 100 parts by weightof the base rubber (a).40. The golf ball according to paragraph 34, wherein the rubbercomposition includes 0.05 parts by weight or greater but 5 parts byweight or less of the organic sulfur compound (e) per 100 parts byweight of the base rubber (a).

EXAMPLES Experiment I Example I-1

A rubber composition was obtained by kneading 100 parts by weight of ahigh-cis polybutadiene (trade name “BR-730”, manufactured by JSRCorporation), 26 parts by weight of zinc diacrylate (trade name“Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5parts by weight of zinc oxide, an appropriate amount of barium sulfate,0.2 parts by weight of 2-thionaphthol, 10 parts by weight of zincstearate, and 0.75 parts by weight of dicumyl peroxide. This rubbercomposition was placed into a mold including upper and lower mold halveseach having a hemispherical cavity, and heated at 170° C. for 25 minutesto obtain a core with a diameter of 39.1 mm. The amount of bariumsulfate was adjusted such that the weight of a golf ball is 45.4 g.

A resin composition was obtained by kneading 40 parts by weight of anionomer resin (the aforementioned “Himilan AM7337”), 40 parts by weightof another ionomer resin (the aforementioned “Himilan AM7329”), 20 partsby weight of a styrene block-containing thermoplastic elastomer (theaforementioned “Rabalon T3221C”), and 6 parts by weight of titaniumdioxide with a twin-screw kneading extruder. The core was placed into amold. The resin composition was injected around the core by injectionmolding to form an inner cover with a thickness of 1.0 mm.

A resin composition was obtained by kneading 5 parts by weight of anionomer resin (the aforementioned “Himilan AM7337”), 10 parts by weightof another ionomer resin (the aforementioned “Himilan 1555”), 55 partsby weight of still another ionomer resin (the aforementioned “HimilanAM7329”), 30 parts by weight of an ethylene-(meth)acrylic acid copolymer(trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUIPOLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide, and 0.2parts by weight of an ultraviolet absorber (trade name “TINUVIN 770”,manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder.The sphere consisting of the core and the inner cover was placed into afinal mold having a large number of pimples on its cavity face. Theresin composition was injected around the sphere by injection molding toform an outer cover with a thickness of 0.8 mm. Dimples having a shapethat is the inverted shape of the pimples were formed on the outercover. A clear paint including a two-component curing type polyurethaneas a base material was applied to the outer cover to obtain a golf ballof Example I-1 with a diameter of 42.7 mm.

Examples 1-2 to I-14 and Comparative Examples I-1 to I-5

Golf balls of Examples 1-2 to I-14 and Comparative Examples I-1 to I-5were obtained in the same manner as Example I-1, except thespecifications of the core, the inner cover, and the outer cover were asshown in Tables 1-7 to I-9 below. The composition of the core is shownin detail in Tables I-1 and I-2 below. The compositions of the innercover and the outer cover are shown in detail in Table I-3 below. Ahardness distribution of the core is shown in Tables I-4 to I-6 below.The golf ball according to Comparative Example I-3 does not have aninner cover.

[Hit with Middle Iron (I #5)]

A 5-iron (trade name “XXIO”, manufactured by SRI Sports Limited, shafthardness: R, loft angle: 24°) was attached to a swing machinemanufactured by Golf Laboratories, Inc. A golf ball was hit under thecondition of a head speed of 35 m/sec. The spin rate was measuredimmediately after the hit. Furthermore, the distance from the launchpoint to the stop point was measured. The average value of data obtainedby 12 measurements is shown in Tables I-7 to I-9 below.

TABLE I-1 Composition of Core (parts by weight) A C D E F M BR-730 100100 100 100 100 100 Sanceler SR 27.0 26.0 27.5 29.5 31.5 25.0 Zinc oxide5 5 5 5 5 5 Barium sulfate Appropriate amount 2-thionaphthol 0.2 0.2 0.20.2 0.2 0.2 Zinc stearate 0 10 20 30 40 — Aluminum stearate — — — — — 10Dicumyl peroxide 0.75 0.75 0.75 0.75 0.75 0.75 Amount of com- 3.86 3.853.86 3.85 3.86 3.83 pressive deformation Dc (mm)

TABLE I-2 Composition of Core (parts by weight) G H I J K L BR-730 100100 100 100 100 100 Sanceler SR 26.5 25.5 25.0 25.5 26.0 25.5 Zinc oxide5 5 5 5 5 5 Barium sulfate Appropriate amount 2-thionaphthol 0.2 0.2 0.20.2 0.2 0.2 Zinc octoate — 2.5 5 — — — Zinc laurate — — — 10 — — Zincmyristate — — — — 5 10 Zinc stearate 0.5 — — — — — Dicumyl peroxide 0.750.75 0.75 0.75 0.75 0.75 Amount of com- 3.86 3.87 3.83 3.85 3.86 3.84pressive deformation Dc (mm)

The details of the compounds listed in Tables I-1 and I-2 are asfollows.

BR-730: a high-cis polybutadiene manufactured by JSR

Corporation (cis-1,4-bond content: 96% by weight, 1,2-vinyl bondcontent: 1.3% by weight, Mooney viscosity (ML₁₊₄ (100° C.)) 55,molecular weight distribution (Mw/Mn): 3)

Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRYCO., LTD. (a product coated with 10% by weight of stearic acid)

Zinc oxide: trade name “Ginrei R” manufactured by Toho Zinc Co., Ltd.

Barium sulfate: trade name “Barium Sulfate BD” manufactured by SakaiChemical Industry Co., Ltd.

2-thionaphthol: a product of Tokyo Chemical Industry Co., Ltd.

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

Aluminum stearate: a product of Mitsuwa Chemicals Co., Ltd.

Dicumyl peroxide: trade name “Percumyl D” manufactured by NOFCorporation

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc laurate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc myristate: a product of NOF Corporation

TABLE I-3 Composition of Cover (parts by weight) C1 I-M1 I-M2 I-M3 I-M4I-M5 Himilan AM7337 5 51 45 40 24 26 Himilan 1555 10 — — — — — HimilanAM7329 55 40 40 40 50 40 NUCREL N1050H 30 — — — — — Rabalon T3221C — 915 20 26 34 Titanium dioxide (A220) 3 6 6 6 6 6 TINUVIN 770 0.2 — — — —— Hardness (JIS C) 92 89 87 85 83 76

TABLE I-4 Hardness Distribution of Core Comp. Comp. Comp. Ex. Ex. Ex.Ex. Ex. Ex . Ex. I-1 I-2 I-1 I-2 I-3 I-3 I-13 Composition of core C C CC C C M H (0)   54.0 54.0 54.0 54.0 54.0 54.0 55.6 H (12.5) 59.8 59.859.8 59.8 59.8 59.8 60.2 H (25)   63.0 63.0 63.0 63.0 63.0 63.0 63.9 H(37.5) 64.6 64.6 64.6 64.6 64.6 64.6 65.4 H (50)   67.0 67.0 67.0 67.067.0 67.0 67.1 H (62.5) 71.8 71.8 71.8 71.8 71.8 71.8 70.9 H (75)   76.076.0 76.0 76.0 76.0 76.0 74.8 H (87.5) 79.5 79.5 79.5 79.5 79.5 79.577.7 Hs 83.0 83.0 83.0 83.0 83.0 83.0 82.3 R² 0.99 0.99 0.99 0.99 0.990.99 0.99

TABLE I-5 Hardness Distribution of Core Comp. Comp. Ex. Ex. Ex. Ex. Ex.Ex. I-4 I-5 I-6 I-7 I-4 I-5 Composition of core C C D E F A H (0)   54.054.0 56.5 59.2 61.9 59.0 H (12.5) 59.8 59.8 59.7 61.5 63.2 64.5 H (25)  63.0 63.0 62.0 63.2 64.3 67.1 H (37.5) 64.6 64.6 62.8 64.0 64.3 67.7 H(50)   67.0 67.0 66.6 66.8 67.0 68.6 H (62.5) 71.8 71.8 73.7 71.0 70.470.6 H (75)   76.0 76.0 75.4 72.1 70.5 74.1 H (87.5) 79.5 79.5 78.2 73.068.5 79.0 Hs 83.0 83.0 81.6 79.1 70.7 83.0 R² 0.99 0.99 0.98 0.96 0.860.94

TABLE I-6 Hardness Distribution of Core Ex. Ex. Ex. Ex. Ex. Ex. I-8 I-9I-10 I-11 I-12 I-14 Composi- H I J K L G tion of core H (0)   53.6 51.454.2 54.9 53.3 57.5 H (12.5) 58.3 57.6 58.2 59.0 58.4 63.5 H (25)   61.761.2 62.1 63.6 62.6 66.6 H (37.5) 65.2 63.8 64.4 67.0 65.5 68.8 H (50)  67.4 67.9 66.4 68.5 67.4 70.0 H (62.5) 71.0 73.8 71.0 70.1 71.8 71.2 H(75)   75.3 77.8 77.0 76.7 77.5 74.8 H (87.5) 80.6 82.0 80.7 80.5 81.378.8 Hs 84.1 84.9 83.3 83.4 84.5 82.9 R² 0.99 0.99 0.99 0.98 0.99 0.96

TABLE I-7 Results of Evaluation Comp. Comp. Comp. Ex. Ex. Ex. Ex. Ex.Ex. Ex. I-1 I-2 I-1 I-2 I-3 I-3 I-13 Core Composition C C C C C C M Acidand/or salt (PHR) 10.0 10.0 10.0 10.0 10.0 10.0 10.0 Diameter (mm) 39.139.1 39.1 39.1 39.1 39.1 39.1 Hs—H(0) 29.0 29.0 29.0 29.0 29.0 29.0 26.7R² 0.99 0.99 0.99 0.99 0.99 0.99 0.99 Amount of compressive 3.85 3.853.85 3.85 3.85 3.85 3.83 deformation Dc (mm) Inner cover CompositionI-M5 I-M4 I-M3 I-M2 I-M1 — I-M2 Thickness (mm) 1.0 1.0 1.0 1.0 1.0 — 1.0Hardness Hi (JIS C) 76.0 83.0 85.0 87.0 89.0 — 87.0 Amount ofcompressive 3.57 3.55 3.55 3.55 3.55 — 3.53 deformation Di (mm) Outercover Composition C1 C1 C1 C1 C1 C1 C1 Thickness (mm) 0.8 0.8 0.8 0.80.8 1.8 0.8 Hardness Ho (JIS C) 92.0 92.0 92.0 92.0 92.0 92.0 92.0Amount of compressive 3.22 3.20 3.20 3.20 3.20 3.25 3.18 deformation Db(mm) Cover total thickness 1.8 1.8 1.8 1.8 1.8 1.8 1.8 (mm) Hi—Hs −7.00.0 2.0 4.0 6.0 — 4.7 Ho—Hi 16.0 9.0 7.0 5.0 3.0 — 5.0 Spin (rpm) 3,8503,820 3,790 3,770 3,750 3,830 3,785 Difference from −50 −80 −110 −130−150 −70 −115 Comp. Ex. 5 Flight distance (m) 150.2 150.4 151.4 151.6151.8 150.3 151.2 Difference from 0.2 0.4 1.4 1.6 1.8 0.3 1.2 Comp. Ex.5

TABLE I-8 Results of Evaluation Comp. Comp. Ex. Ex. Ex. Ex. Ex. Ex. I-4I-5 I-6 I-7 I-4 I-5 Core Composition C C D E F A Acid and/or 10.0 10.020.0 30.0 40.0 0.0 salt (PHR) Diameter (mm) 38.5 37.9 39.1 39.1 39.139.1 Hs—H(0) 28.5 28.1 25.1 19.9 8.8 24.0 R² 0.99 0.99 0.98 0.96 0.860.94 Amount of 3.85 3.85 3.86 3.85 3.86 3.86 compressive deformation Dc(mm) Inner cover Composition I-M3 I-M3 I-M3 I-M3 I-M3 I-M3 Thickness(mm) 1.0 1.0 1.0 1.0 1.0 1.0 Hardness Hi 85.0 85.0 85.0 85.0 85.0 85.0(JIS C) Amount of 3.55 3.55 3.56 3.60 3.66 3.56 compressive deformationDi (mm) Outer cover Composition C1 C1 C1 C1 C1 C1 Thickness (mm) 1.1 1.40.8 0.8 0.8 0.8 Hardness Ho 92.0 92.0 92.0 92.0 92.0 92.0 (JIS C) Amountof 3.18 3.16 3.21 3.25 3.31 3.21 compressive deformation Db (mm) Covertotal 2.1 2.4 1.8 1.8 1.8 1.8 thickness (mm) Hi—Hs 1.5 2.9 3.4 5.9 14.32.0 Ho—Hi 7.0 7.0 7.0 7.0 7.0 7.0 Spin (rpm) 3,795 3,775 3,780 3,8004,000 3,900 Difference from −105 −125 −120 −100 100 — Comp. Ex. 5 Flightdistance 151.1 151.3 151.2 151.0 149.4 150.0 (m) Difference 1.1 1.3 1.21.0 -0.6 — from Comp. Ex. 5

TABLE I-9 Results of Evaluation Ex. Ex. Ex. Ex. Ex. Ex. I-8 I-9 I-10I-11 I-12 I-14 Core Composition H I J K L G Acid and/or salt 2.5 5.010.0 5.0 10.0 0.5 (PHR) Diameter (mm) 39.1 39.1 39.1 39.1 39.1 39.1 Hs—H(0) 30.5 33.5 29.1 28.5 31.2 25.4 R² 0.99 0.99 0.99 0.98 0.99 0.96Amount of 3.87 3.83 3.85 3.86 3.84 3.86 compres- sive deformation Dc(mm) Inner cover Composition I-M1 I-M1 I-M1 I-M1 I-M1 I-M1 Thickness(mm) 1.0 1.0 1.0 1.0 1.0 1.0 Hardness 89.0 89.0 89.0 89.0 89.0 89.0 Hi(JIS C) Amount of 3.57 3.53 3.55 3.56 3.54 3.56 compres- sivedeformation Di (mm) Outer cover Composition C1 C1 C1 C1 C1 C1 Thickness0.8 0.8 0.8 0.8 0.8 0.8 (mm) Hardness Ho 92.0 92.0 92.0 92.0 92.0 92.0(JIS C) Amount of 3.22 3.18 3.20 3.21 3.19 3.21 compressive deformationDb (mm) Cover total 1.8 1.8 1.8 1.8 1.8 1.8 thickness (mm) Hi—Hs 4.9 4.15.7 5.6 4.5 6.1 Ho—Hi 3.0 3.0 3.0 3.0 3.0 3.0 Spin (rpm) 3735 3710 37453765 3740 3850 Difference from −165 −190 −155 −135 −160 −50 Comp. Ex. 5Flight distance 151.9 152.2 151.7 151.5 152.0 150.5 (m) Difference from1.9 2.2 1.7 1.5 2.0 0.5 Comp. Ex. 5

As shown in Tables I-7 to I-9, the golf balls according to Examples haveexcellent flight performance upon a shot with a middle iron. From theresults of evaluation, advantages of the present invention are clear.

Experiment II Example II-1

A rubber composition was obtained by kneading 100 parts by weight of ahigh-cis polybutadiene (trade name “BR-730”, manufactured by JSRCorporation), 26 parts by weight of zinc diacrylate (trade name“Sanceler SR”, manufactured by SANSHIN CHEMICAL INDUSTRY CO., LTD.), 5parts by weight of zinc oxide, an appropriate amount of barium sulfate,0.2 parts by weight of 2-thionaphthol, 10 parts by weight of zincstearate, and 0.75 parts by weight of dicumyl peroxide. This rubbercomposition was placed into a mold including upper and lower mold halveseach having a hemispherical cavity, and heated at 170° C. for 25 minutesto obtain a core with a diameter of 39.1 mm. The amount of bariumsulfate was adjusted such that the weight of a golf ball is 45.4 g.

A resin composition was obtained by kneading 24 parts by weight of anionomer resin (the aforementioned “Himilan AM7337”), 50 parts by weightof another ionomer resin (the aforementioned “Himilan AM7329”), 26 partsby weight of a styrene block-containing thermoplastic elastomer (theaforementioned “Rabalon T3221C”), and 6 parts by weight of titaniumdioxide with a twin-screw kneading extruder. The core was placed into amold. The resin composition was injected around the core by injectionmolding to form an inner cover with a thickness of 1.0 mm.

A resin composition was obtained by kneading 5 parts by weight of anionomer resin (the aforementioned “HimilanAM7337”), 10 parts by weightof another ionomer resin (the aforementioned “Himilan 1555”), 55 partsby weight of still another ionomer resin (the aforementioned “HimilanAM7329”), 30 parts by weight of an ethylene-(meth)acrylic acid copolymer(trade name “NUCREL N1050H”, manufactured by Du Pont-MITSUIPOLYCHEMICALS Co., Ltd.), 3 parts by weight of titanium dioxide, and 0.2parts by weight of an ultraviolet absorber (trade name “TINUVIN 770”,manufactured by Ciba Japan K.K.) with a twin-screw kneading extruder.The sphere consisting of the core and the inner cover was placed into afinal mold having a large number of pimples on its cavity face. Theresin composition was injected around the sphere by injection molding toform an outer cover with a thickness of 0.8 mm. Dimples having a shapethat is the inverted shape of the pimples were formed on the outercover. A clear paint including a two-component curing type polyurethaneas a base material was applied to the outer cover to obtain a golf ballof Example II-1 with a diameter of 42.7 mm.

Examples II-2 to II-14 and Comparative Examples II-1 to II-5

Golf balls of Examples II-2 to II-14 and Comparative Examples II-1 toII-5 were obtained in the same manner as Example II-1, except thespecifications of the core, the inner cover, and the outer cover were asshown in Tables II-7 to II-9 below. The composition of the core is shownin detail in Tables II-1 and II-2 below. The compositions of the innercover and the outer cover are shown in detail in Table II-3 below. Ahardness distribution of the core is shown in Tables II-4 to II-6 below.The golf ball according to Comparative Example II-2 does not have aninner cover.

[Hit with Driver (W #1)]

A driver (trade name “XXIO”, manufactured by SRI Sports Limited, shafthardness: R, loft angle: 10.5°) was attached to a swing machinemanufactured by Golf Laboratories, Inc. A golf ball was hit under thecondition of a head speed of 40 m/sec. The spin rate was measuredimmediately after the hit. Furthermore, the distance from the launchpoint to the stop point was measured. The average value of data obtainedby 12 measurements is shown in Tables II-7 to II-9 below.

[Feel at Impact]

Ten golf players hit golf balls with drivers and were asked about feelat impact. The evaluation was categorized as follows on the basis of thenumber of golf players who answered, “the feel at impact was favorable”.

A: 8 or more

B: 6 to 7

C: 4 to 5

D: 3 or less

The results are shown in Tables II-7 to II-9 below.

TABLE II-1 Composition of Core (parts by weight) A C D E F M BR-730 100100 100 100 100 100 Sanceler SR 27.0 26.0 27.5 29.5 31.5 25.0 Zinc oxide5 5 5 5 5 5 Barium sulfate Appropriate amount 2-thionaphthol 0.2 0.2 0.20.2 0.2 0.2 Zinc stearate 0 10 20 30 40 — Aluminum stearate — — — — — 10Dicumyl peroxide 0.75 0.75 0.75 0.75 0.75 0.75 Amount of 3.86 3.85 3.863.85 3.86 3.83 compressive deformation Dc (mm)

TABLE II-2 Composition of Core (parts by weight) G H I J K L BR-730 100100 100 100 100 100 Sanceler SR 26.5 25.5 25.0 25.5 26.0 25.5 Zinc oxide5 5 5 5 5 5 Barium sulfate Appropriate amount 2-thionaphthol 0.2 0.2 0.20.2 0.2 0.2 Zinc octoate — 2.5 5 — — — Zinc laurate — — — 10 — — Zincmyristate — — — — 5 10 Zinc stearate 0.5 — — — — — Dicumyl 0.75 0.750.75 0.75 0.75 0.75 peroxide Amount of 3.86 3.87 3.83 3.85 3.86 3.84compressive deformation Dc (mm)

The details of the compounds listed in Tables II-1 and II-2 are asfollows.

BR-730: a high-cis polybutadiene manufactured by JSR Corporation(cis-1,4-bond content: 96% by weight, 1,2-vinyl bond content: 1.3% byweight, Mooney viscosity (ML₁₊₄(100° C.)): 55, molecular weightdistribution (Mw/Mn): 3)

Sanceler SR: zinc diacrylate manufactured by SANSHIN CHEMICAL INDUSTRYCO., LTD. (a product coated with 10% by weight of stearic acid)

Zinc oxide: trade name “Ginrei R” manufactured by Toho Zinc Co., Ltd.

Barium sulfate: trade name “Barium Sulfate ED” manufactured by SakaiChemical Industry Co., Ltd.

2-thionaphthol: a product of Tokyo Chemical Industry Co., Ltd.

Zinc stearate: a product of Wako Pure Chemical Industries, Ltd.

Aluminum stearate: a product of Mitsuwa Chemicals Co., Ltd.

Dicumyl peroxide: trade name “Percumyl D” manufactured by NOFCorporation

Zinc octoate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc laurate: a product of Mitsuwa Chemicals Co., Ltd.

Zinc myristate: a product of NOF Corporation

TABLE II-3 Composition of Cover (parts by weight) C1 II-M1 II-M2 II-M3II-M4 II-M5 Himilan AM7337 5 45 24 26 30 26 Himilan 1555 10 — — — — —Himilan AM7329 55 40 50 40 30 26 NUCREL N1050H 30 — — — — — RabalonT3221C — 15 26 34 40 48 Titanium dioxide (A220) 3 6 6 6 6 6 TINUVIN 7700.2 — — — — — Hardness (JIS C) 92 87 83 76 71 65

TABLE II-4 Hardness Distribution of Core Ex. II-1 Ex. II-2 Ex. II-3 Ex.II-4 Comp. Ex. II-1 Comp. Ex. II-2 Ex. II-14 Composition of core C C C CC C M H (0)   54.0 54.0 54.0 54.0 54.0 54.0 55.6 H (12.5) 59.8 59.8 59.859.8 59.8 59.8 60.2 H (25)   63.0 63.0 63.0 63.0 63.0 63.0 63.9 H (37.5)64.6 64.6 64.6 64.6 64.6 64.6 65.4 H (50)   67.0 67.0 67.0 67.0 67.067.0 67.1 H (62.5) 71.8 71.8 71.8 71.8 71.8 71.8 70.9 H (75)   76.0 76.076.0 76.0 76.0 76.0 74.8 H (87.5) 79.5 79.5 79.5 79.5 79.5 79.5 77.7 Hs83.0 83.0 83.0 83.0 83.0 83.0 82.3 R² 0.99 0.99 0.99 0.99 0.99 0.99 0.99

TABLE II-5 Hardness Distribution of Core Comp. Comp. Ex. Ex. Ex. Ex. Ex.Ex. II-5 II-6 II-7 II-8 II-3 II-4 Composi- C C D E F A tion of core H(0)   54.0 54.0 56.5 59.2 61.9 59.0 H (12.5) 59.8 59.8 59.7 61.5 63.264.5 H (25)   63.0 63.0 62.0 63.2 64.3 67.1 H (37.5) 64.6 64.6 62.8 64.064.3 67.7 H (50)   67.0 67.0 66.6 66.8 67.0 68.6 H (62.5) 71.8 71.8 73.771.0 70.4 70.6 H (75)   76.0 76.0 75.4 72.1 70.5 74.1 H (87.5) 79.5 79.578.2 73.0 68.5 79.0 Hs 83.0 83.0 81.6 79.1 70.7 83.0 R² 0.99 0.99 0.980.96 0.86 0.94

TABLE II-6 Hardness Distribution of Core Comp. Ex. Ex. Ex. Ex. Ex. Ex.II-9 II-10 II-11 II-12 II-13 II-5 Composition H I J K L G of core H(0)   53.6 51.4 54.2 54.9 53.3 57.5 H (12.5) 58.3 57.6 58.2 59.0 58.463.5 H (25)   61.7 61.2 62.1 63.6 62.6 66.6 H (37.5) 65.2 63.8 64.4 67.065.5 68.8 H (50)   67.4 67.9 66.4 68.5 67.4 70.0 H (62.5) 71.0 73.8 71.070.1 71.8 71.2 H (75)   75.3 77.8 77.0 76.7 77.5 74.8 H (87.5) 80.6 82.080.7 80.5 81.3 78.8 Hs 84.1 84.9 83.3 83.4 84.5 82.9 R² 0.99 0.99 0.990.98 0.99 0.96

TABLE II-7 Results of Evaluation Comp. Comp. Ex. Ex. Ex. Ex. Ex. Ex. Ex.II-1 II-1 II-2 II-3 II-4 II-2 II-14 Core Composi- tion C C C C C C MAcid and/or salt (PHR) 10.0 10.0 10.0 10.0 10.0 10.0 10.0 Diameter (mm)39.1 39.1 39.1 39.1 39.1 39.1 39.1 Hs—H(0) 29.0 29.0 29.0 29.0 29.0 29.026.7 R² 0.99 0.99 0.99 0.99 0.99 0.99 0.99 Amount of compressive 3.853.85 3.85 3.85 3.85 3.85 3.83 deformation Dc (mm) Inner coverComposition II-M1 II-M2 II-M3 II-M4 II-M5 — II-M4 Thickness (mm) 1.0 1.01.0 1.0 1.0 — 1.0 Hardness Hi (JIS C) 87.0 83.0 76.0 71.0 65.0 — 71.0Amount of compressive 3.50 3.55 3.57 3.58 3.60 — 3.56 deformation Di(mm) Outer cover Composition C1 C1 C1 C1 C1 C1 C1 Thickness (mm) 0.8 0.80.8 0.8 0.8 1.8 0.8 Hardness Ho (JIS C) 92.0 92.0 92.0 92.0 92.0 92.092.0 Amount of compressive 3.15 3.20 3.22 3.23 3.25 3.25 3.21deformation Db (mm) Cover total thickness 1.8 1.8 1.8 1.8 1.8 1.8 1.8(mm) Hs—Hi −4.0 0.0 7.0 12.0 18.0 — 11.3 Ho—Hi 5.0 9.0 16.0 21.0 27.0 —21.0 Spin (rpm) 2,455 2,430 2,420 2,410 2,400 2,460 2,430 Differencefrom −45 −70 −80 −90 −100 −40 −70 Comp. Ex. 4 Flight distance (m) 200.7201.7 201.9 202.1 202.5 200.4 201.6 Difference from 0.7 1.7 1.9 2.1 2.50.4 1.6 Comp. Ex. 4 Feel at impact D B A A C D B

TABLE II-8 Results of Evaluation Comp. Comp. Ex. Ex. Ex. Ex. Ex. Ex.II-5 II-6 II-7 II-8 II-3 II-4 Core Composition C C D E F A Acid and/orsalt (PHR) 10.0 10.0 20.0 30.0 40.0 0.0 Diameter (mm) 38.5 37.9 39.139.1 39.1 39.1 Hs—H (0) 28.5 28.1 25.1 19.9 8.8 24.0 R² 0.99 0.99 0.980.96 0.86 0.94 Amount of compressive 3.85 3.85 3.86 3.85 3.86 3.86deformation Dc (mm) Inner cover Composition II-M3 II-M3 II-M4 II-M4II-M2 II-M2 Thickness (mm) 1.0 1.0 1.0 1.0 1.0 1.0 Hardness Hi (JIS C)76.0 76.0 71.0 71.0 83.0 83.0 Amount of compressive 3.57 3.57 3.59 3.583.66 3.56 deformation Di (mm) Outer cover Composition C1 C1 C1 C1 C1 C1Thickness (mm) 1.1 1.4 0.8 0.8 0.8 0.8 Hardness Ho (JIS C) 92.0 92.092.0 92.0 92.0 92.0 Amount of compressive deformation 3.20 3.18 3.243.23 3.31 3.21 Db (mm) Cover total thickness 2.1 2.4 1.8 1.8 1.8 1.8(mm) Hs—Hi 6.5 6.1 10.6 8.1 12.3 0.0 Ho—Hi 16.0 16.0 21.0 21.0 9.0 9.0Spin (rpm) 2,435 2,445 2,440 2,450 2,560 2,500 Difference from −65 −55−60 −50 60 — Comp. Ex. 4 Flight distance (m) 201.5 201.3 201.4 201.1198.9 200.0 Difference from 1.5 1.3 1.4 1.1 −1.1 — Comp. Ex. 4 Feel atimpact B C B C D D

Table II-9 Results of Evaluation Comp. Ex. Ex. Ex. Ex. Ex. Ex. II-9II-10 II-11 II-12 II-13 II-5 Core Composition H I J K L G Acid and/orsalt 2.5 5.0 10.0 5.0 10.0 0.5 (PHR) Diameter (mm) 39.1 39.1 39.1 39.139.1 39.1 Hs—H (0) 30.5 33.5 29.1 28.5 31.2 25.4 R² 0.99 0.99 0.99 0.980.99 0.96 Amount of 3.87 3.83 3.85 3.86 3.84 3.86 compressivedeformation Dc (mm) Inner cover Composition II-M2 II-M2 II-M2 II-M2II-M2 II-M2 Thickness (mm) 1.0 1.0 1.0 1.0 1.0 1.0 Hardness Hi 83.0 83.083.0 83.0 83.0 83.0 (JIS C) Amount of 3.57 3.53 3.55 3.56 3.54 3.56compressive deformation Di (mm) Outer cover Composition C1 C1 C1 C1 C1C1 Thickness (mm) 0.8 0.8 0.8 0.8 0.8 0.8 Hardness Ho 92.0 92.0 92.092.0 92.0 92.0 (JIS C) Amount of 3.22 3.18 3.20 3.21 3.19 3.21compressive deformation Db (mm) Cover total 1.8 1.8 1.8 1.8 1.8 1.8thickness (mm) Hs—Hi 1.1 1.9 0.3 0.4 1.5 −0.1 Ho—Hi 9.0 9.0 9.0 9.0 9.09.0 Spin (rpm) 2415 2380 2405 2425 2385 2480 Difference from −85 −120−95 −75 −115 −20 Comp. Ex. 4 Flight distance (m) 202.0 202.7 202.3 201.8202.6 200.6 Difference from 2.0 2.7 2.3 1.8 2.6 0.6 Comp. Ex. 4 Feel atimpact B A A B A D

As shown in Tables II-7 to II-9, the golf balls according to Exampleshave excellent flight performance and excellent feel at impact upon ashot with a driver. From the results of evaluation, advantages of thepresent invention are clear.

The golf ball according to the present invention can be used for playinggolf on golf courses and practicing at driving ranges. The abovedescriptions are merely for illustrative examples, and variousmodifications can be made without departing from the principles of thepresent invention.

What is claimed is:
 1. A golf ball comprising a spherical core and acover covering the core and including two or more layers, wherein whendistances (%) from a central point of the core to nine points and JIS-Chardnesses at the nine points, which nine points are obtained bydividing a region from the central point of the core to a surface of thecore at intervals of 12.5% of a radius of the core, are plotted in agraph, R² of a linear approximation curve obtained by a least-squaremethod is equal to or greater than 0.95, and a JIS-C hardness Hi of aninnermost layer of the cover is greater than a JIS-C hardness Hs at thesurface of the core.
 2. The golf ball according to claim 1, wherein adifference (Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0)at the central point of the core is equal to or greater than
 15. 3. Thegolf ball according to claim 1, wherein a difference (Hi−Hs) between thehardness Hi and the hardness Hs is equal to or greater than 1 but equalto or less than
 5. 4. The golf ball according to claim 1, wherein adifference (Ho−Hi) between a JIS-C hardness Ho of an outermost layer ofthe cover and the JIS-C hardness Hi of the innermost layer of the coveris equal to or greater than 2 but equal to or less than
 10. 5. The golfball according to claim 1, wherein the core is formed by a rubbercomposition being crosslinked, the rubber composition includes: (a) abase rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator;and (d) an acid and/or a salt, and the co-crosslinking agent (b) is:(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.
 6. The golf ball according to claim 5, wherein an amountof the acid and/or the salt (d) is equal to or greater than 0.5 parts byweight but equal to or less than 40 parts by weight, per 100 parts byweight of the base rubber (a).
 7. The golf ball according to claim 5,wherein the acid and/or the salt (d) is a carboxylic acid and/or a saltthereof (d1).
 8. The golf ball according to claim 7, wherein a carbonnumber of a carboxylic acid component of the carboxylic acid and/or thesalt thereof (d1) is equal to or greater than 1 but equal to or lessthan
 30. 9. The golf ball according to claim 7, wherein the carboxylicacid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof.10. The golf ball according to claim 7, wherein the carboxylic acidand/or the salt thereof (d1) is a zinc salt of a carboxylic acid.
 11. Agolf ball comprising a spherical core and a cover covering the core andincluding two or more layers, wherein when distances (%) from a centralpoint of the core to nine points and JIS-C hardnesses at the ninepoints, which nine points are obtained by dividing a region from thecentral point of the core to a surface of the core at intervals of 12.5%of a radius of the core, are plotted in a graph, R² of a linearapproximation curve obtained by a least-square method is equal to orgreater than 0.95, and a JIS-C hardness Hi of an innermost layer of thecover is equal to or less than a JIS-C hardness Hs at the surface of thecore.
 12. The golf ball according to claim 11, wherein a difference(Hs−H(0)) between the hardness Hs and a JIS-C hardness H(0) at thecentral point of the core is equal to or greater than
 15. 13. The golfball according to claim 11, wherein a difference (Hs−Hi) between thehardness Hs and the hardness Hi is equal to or greater than 1 but equalto or less than
 20. 14. The golf ball according to claim 11, wherein adifference (Ho−Hi) between a JIS-C hardness Ho of an outermost layer ofthe cover and the JIS-C hardness Hi of the innermost layer of the coveris equal to or greater than 5 but equal to or less than
 30. 15. The golfball according to claim 11, wherein the core is formed by a rubbercomposition being crosslinked, the rubber composition includes: (a) abase rubber; (b) a co-crosslinking agent; (c) a crosslinking initiator;and (d) an acid and/or a salt, and the co-crosslinking agent (b) is:(b1) an α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms; or(b2) a metal salt of an α,β-unsaturated carboxylic acid having 3 to 8carbon atoms.
 16. The golf ball according to claim 15, wherein an amountof the acid and/or the salt (d) is equal to or greater than 0.5 parts byweight but equal to or less than 40 parts by weight, per 100 parts byweight of the base rubber (a).
 17. The golf ball according to claim 15,wherein the acid and/or the salt (d) is a carboxylic acid and/or a saltthereof (d1).
 18. The golf ball according to claim 17, wherein a carbonnumber of a carboxylic acid component of the carboxylic acid and/or thesalt thereof (d1) is equal to or greater than 1 but equal to or lessthan
 30. 19. The golf ball according to claim 17, wherein the carboxylicacid and/or the salt thereof (d1) is a fatty acid and/or a salt thereof.20. The golf ball according to claim 17, wherein the carboxylic acidand/or the salt thereof (d1) is a zinc salt of a carboxylic acid.